Brake Upgrade Master Cylinder Question

[Luisv]

This thread comes as a result of a brake upgrade I did on the Gen 2 Viper. The details can be seen on this thread:

http://driveviper.com/forums/threads...de-Gen-2-Viper

The cliff note version is that I upgraded the fronts to 14" rotors with SRT10 Brembo 44/40 calipers. I then moved the front calipers I had to the rear giving me 13" rotors with 42/38 calipers there. The net result is a 2002 Viper on 7 year old tires (now replaced - I am writing this as I am having the car aligned) that stopped in 103 feet. The upgrade is great! The reason for this post is to see if I can make the brakes even better!

The issue/question/consideration I have comes as a result of what I observed after I made the upgrade. What is happening is that, when I get on the brakes very hard, essentially on the extreme, the ABS engages at the front before I can get the rear to go to ABS. In fact, I can't get the rear to lock at all. Typically, what I have normally beenb acustomed to is a rear locking up first.

The basic question is: Could I benefit from a new master cylinder that might allow me to lock the rears before or, at least, at the time as the fronts?

On other threads, the discussion came up re: the front to rear bias. I really could not get ann answer that was agreed upon or that made sense to me so I decided to start digging. So, as background, I figure I'd provide confirmed details about how the car comes from factory. Here is what we have:

-The factory master cylinder is a tandem master. Nothing mystic here as most modern four wheel disc cars are setup this way.
- The master cylinder is a 1.125" (1 1/8") bore with a 1.54" stroke on 2001 and 2002 Vipers. I can confirm the bore is the same on Gen 3 and 4. I would assume the stroke is the same.
- The front to rear bias is 65/35. I've gotten all sorts of answers to that question ranging from 50/50 to 85/15. This 65/35 is confirmed.
- There is no proportioning block on Gen 2 Vipers. The proportioning is handled at the master cylinder.
- On Gen 2 ABS Vipers (I would venture to say this is the same in later versions as well) the ABS control unit is a 2-IN / 3-OUT setup. This means that the ABS unit gets two lines coming from the master cylinder (2-IN) and then has two outputs for the front and one for the rear. (3-OUT) We do, however, have ABS sensors on each wheel. What this means is that if either of the rears lock, both will have ABS applied. The fronts are independent, if one locks, only that wheel gets ABS.
- The pedal ratio is 3.48:1 This means that when you apply 100 lbs of pressure on the brake pedal, you are applying 348 lbs of pressure on the master cylinder linkage.

So the bottom line... The pedal "feel" is great. I have a bit more travel, as expected and told by a few folks before I did the upgrade. The extra travel is nice because you have more ability to modulate braking. In the original setup it was very little travel before the pedal was maxed out and would not go down further.

First Option: I was considering was going to a slightly smaller bore, perhaps 1.0625 ( 1-1/16") or a 1" bore. This would increase pressure. If I do that, I might be able to lock the rears. Right now, it feels like I am not fully utilizing those 335's.
Second Option: A master that is 55/45 or 50/50, if that is even available.
Third Option: Dual Master cylinder with a balance bar

As FYI for those wondering that may not know: The smaller the bore the higher the pressure at the caliper. The relationship is inverse. Think of it as a pressure washer. If you have a 3000 PSI washer and put a fan tip on it the pressure is split out over the width of the fan shape. If you have the "laser" nozzle the 3000 PSI is focused on a sharp point. So a smaller bore... higher concentration of force from the pedal... this equates to higher PSI.

Let the opinions fly folks....

[95Viper]

I can tell u from my 1200hp pro-street chevy that I had similar issues due to several factors one being the tire size difference,,,,, I ended up going with a full wilwood set up... they were a big help with all the math to determine the volume/pressure issues. I've been thinking about getting a m/cylinder from them for my Gen1 to improve pedal feel and better balance. I'm sure they could help. Look forward to your results.

[Luisv]

I can tell u from my 1200hp pro-street chevy that I had similar issues due to several factors one being the tire size difference,,,,, I ended up going with a full wilwood set up... they were a big help with all the math to determine the volume/pressure issues. I've been thinking about getting a m/cylinder from them for my Gen1 to improve pedal feel and better balance. I'm sure they could help. Look forward to your results.

I purchased a Wilwood kit for a '77 Vette I am resto-modding. They were real helpful. I called them about this issue. What they told me was that they only make a 70/30 tandem master with a 1.125" bore. Actually they called it a 2:1 setup. This would mean a 66.7/33.3 setup... Bottom line... the same thing.

I was thinking of looking to Centric/Stoptech next.

[Sybil TF]

I ended up going with a full wilwood set up... they were a big help with all the math to determine the volume/pressure issues. getting a m/cylinder

My car came with a full Wilwood front and rear with a proportioning valve. My brakes are spectacular! And with the turn of a knob I can adjust the rears to come into play as much as I like.

[Dan Cragin]

The piston sizes in the rear are way to big. You should consider using a SRT10 rear caliper, Partsrack has a kit. This will correct the bias issue.

[Luisv]

The piston sizes in the rear are way to big. You should consider using a SRT10 rear caliper, Partsrack has a kit. This will correct the bias issue.

Dan, I don't think that's the issue. The piston sizes are identical. The SRT10 rear caliper is a 42mm leading and 38mm trailing piston. The fronts from the Gen 2 GTS are 42mm / 38mm as well. From a piston area/size standpoint, they are the same.

The only difference between my setup and the setup on the SRT10 is simply the rotor size. I have 13" and the SRT10 has 14" rears.

[djos83]

Dan, I don't think that's the issue. The piston sizes are identical. The SRT10 rear caliper is a 42mm leading and 38mm trailing piston. The fronts from the Gen 2 GTS are 42mm / 38mm as well. From a piston area/size standpoint, they are the same.

The only difference between my setup and the setup on the SRT10 is simply the rotor size. I have 13" and the SRT10 has 14" rears.

Hi Luis - I have a similar setup as yours on my 97 GTS race car. I think the problem is your piston volume in the rear is much much more than your stock Gen 2 ABS 42mm single rear piston caliper, which is the same problem that I have. Your new rear caliper (which was your front caliper) requires greater volume to fill, which is why your brake pedal travel has increased. If you decrease your MC bore size, your pedal travel will increase even further. This will result in even more pedal travel and more sensitivity at the end of the travel, and hence, imo, less control at the limit. Your front brakes will still lockup before your rear calipers because they will fill before your rear calipers. If your car was non-ABS, I would say go with a dual MC with balance bar. You could then choose the MC bore sizes appropriately. However, given your car is ABS, I'm not sure. Personally, I'm going back to stock rear calipers to reduce pedal travel and regain some modulation and feel.

[Luisv]

Hi Luis - I have a similar setup as yours on my 97 GTS race car. I think the problem is your piston volume in the rear is much much more than your stock Gen 2 ABS 42mm single rear piston caliper, which is the same problem that I have. Your new rear caliper (which was your front caliper) requires greater volume to fill, which is why your brake pedal travel has increased. If you decrease your MC bore size, your pedal travel will increase even further. This will result in even more pedal travel and more sensitivity at the end of the travel, and hence, imo, less control at the limit. Your front brakes will still lockup before your rear calipers because they will fill before your rear calipers. If your car was non-ABS, I would say go with a dual MC with balance bar. You could then choose the MC bore sizes appropriately. However, given your car is ABS, I'm not sure. Personally, I'm going back to stock rear calipers to reduce pedal travel and regain some modulation and feel.

Thanks for the reply... This was very helpful and your right on the pedal travel is a bit longer. My biggest concern was the fact that the front lock in the extreme before the rears. However, having said that, I've got a few hundred miles and a couple of hours behind the wheel.

As I expected, I find I like the additional travel. I figure it's a matter of taste and what I've been used to over the years. Personally, I feel like I have more of an ability to modulate/control the braking. I am getting used to the setup and I am liking it more and more.

I do like the idea of the twin master and balance bar. We looked into that for a Vette we are restoring. I will do some more homework on that. The ABS is a two in / three out setup. I have to see how that will work. I had not considered that. Lets see what I can dig up. The first thing that comes to mind is the lack of space. The master is a great deal tighter against the front wheel well than I thought. Regardless.... lets see what I can dig up.

Thanks again...

[djos83]

Thanks for the reply... This was very helpful and your right on the pedal travel is a bit longer. My biggest concern was the fact that the front lock in the extreme before the rears. However, having said that, I've got a few hundred miles and a couple of hours behind the wheel.

As I expected, I find I like the additional travel. I figure it's a matter of taste and what I've been used to over the years. Personally, I feel like I have more of an ability to modulate/control the braking. I am getting used to the setup and I am liking it more and more.

I do like the idea of the twin master and balance bar. We looked into that for a Vette we are restoring. I will do some more homework on that. The ABS is a two in / three out setup. I have to see how that will work. I had not considered that. Lets see what I can dig up. The first thing that comes to mind is the lack of space. The master is a great deal tighter against the front wheel well than I thought. Regardless.... lets see what I can dig up.

Thanks again...

Another thought, before you do do anything with the master cylinder, maybe you should try different brake pad compounds. What pads are you running now?

[ACR_Matt]

Any new developments on this Luis? I have a non-abs 2000 model so I'll be installing a proportioning valve to solve the front-rear bias issue when I finish the SRT10 caliper swap. Is there a way to install a prop valve on abs equipped cars without the system having a mental breakdown? Just a thought.

-Matt

[Blue96GTS]

Rehashing this thread, as my questions are relevant to the topic(s). Thanks in advance for your replies!

I'm getting ready to replace my Wilwood setup (previous owner installed) with a StopTech 4 pot front/rear solution. Existing setup also includes an aftermarket proportioning valve for adjusting rear bias. 96 GTS with no ABS.

My questions:

Q1: Do I need this proportioning valve? Other threads on other boards seem to indicate that the proportioning valve should be eliminated (or spring removed), as it's only decreasing the pressure that could be applied to the rears - perhaps unnecessarily so. Other threads seem to indicate that bias could be adjusted/tweaked with pads. So, do I need the valve, or not?

Q2: How can I effectively determine if my bias is correctly setup?? Typically one would get a buddy to stand outside the car while the driver goes by and locks up the brakes. This is cool, but I don't really want to be flat-spotting my new tires, attempting to tweak the bias. Brake testing on post-snow, sand laden Colorado roads seems like a better option for preserving the tires, but may still result in a Darwin award. Not good.

Thoughts??

[Luisv]

Rehashing this thread, as my questions are relevant to the topic(s). Thanks in advance for your replies!

I'm getting ready to replace my Wilwood setup (previous owner installed) with a StopTech 4 pot front/rear solution. Existing setup also includes an aftermarket proportioning valve for adjusting rear bias. 96 GTS with no ABS.

My questions:

Q1: Do I need this proportioning valve? Other threads on other boards seem to indicate that the proportioning valve should be eliminated (or spring removed), as it's only decreasing the pressure that could be applied to the rears - perhaps unnecessarily so. Other threads seem to indicate that bias could be adjusted/tweaked with pads. So, do I need the valve, or not?

Q2: How can I effectively determine if my bias is correctly setup?? Typically one would get a buddy to stand outside the car while the driver goes by and locks up the brakes. This is cool, but I don't really want to be flat-spotting my new tires, attempting to tweak the bias. Brake testing on post-snow, sand laden Colorado roads seems like a better option for preserving the tires, but may still result in a Darwin award. Not good.

Thoughts??

On Q1. I would say I'd keep the proportioning valve. If nothing else, you wil have adjustability. As for not having it because it decreases rear pressure... well that's the point. You want to balance the brakes front to back with it. In cars without ABS you can use the bias adjustment to avoid locking the rears which throws the car around. You want to have rears lock later than fronts.

Can you modulate with the pads. Sure.... but what if you get it wrong? Not everybody drives the same, not everybody can control braking the same. What works for one person may not wok for another. I'd hate to see you try what works for person A in a forum rather than to setup the car for you. With the valve, if you get it wrong, you turn a knob until it works for you.

On Q2. You don't need to lock tires and slide for 100 feet to set bias. You do it by testing and just locking brakes and realease. No flat spotting needed. All you want is to set the bias such that the rears lock after the fronts. This is the best way to control the car in a hard braking situation in on road driving. Just test in a large lot so that you can just lock the tires and then release and roll out without hitting something. It's not hugely difficult.

[duffman04]

Rehashing this thread, as my questions are relevant to the topic(s). Thanks in advance for your replies!

I'm getting ready to replace my Wilwood setup (previous owner installed) with a StopTech 4 pot front/rear solution. Existing setup also includes an aftermarket proportioning valve for adjusting rear bias. 96 GTS with no ABS.

My questions:

Q1: Do I need this proportioning valve? Other threads on other boards seem to indicate that the proportioning valve should be eliminated (or spring removed), as it's only decreasing the pressure that could be applied to the rears - perhaps unnecessarily so. Other threads seem to indicate that bias could be adjusted/tweaked with pads. So, do I need the valve, or not?

Q2: How can I effectively determine if my bias is correctly setup?? Typically one would get a buddy to stand outside the car while the driver goes by and locks up the brakes. This is cool, but I don't really want to be flat-spotting my new tires, attempting to tweak the bias. Brake testing on post-snow, sand laden Colorado roads seems like a better option for preserving the tires, but may still result in a Darwin award. Not good.

Thoughts??

Q1: Without a doubt keep the proportioning valve/brake bias controller. Most of the cars I used to race had electronically controlled brake bias controllers, with the ideal setup around 60/40 - f/r depending on the day and track conditions. Honestly, like Luisv said, there is very little utility in trying to adjust brake bias with different brake pad compounds. Especially when track conditions change, tire wear occurs. Brake bias can generally be a set it and forget it type of thing, but the need for a couple percent front or a couple percent rear is common and you'd never have the option without a proper bias controller.

Q2: The initial setup will be the most difficult, just to get it in the optimal range. Under hard braking, and I mean hard at the limit of locking up braking, if the rear starts to feel loose thats usually an indication to start moving a couple percent back to the rear until it starts to stabilize. Obviously other factors come into play like the aero grip you're generating or losing, the type of ABS system you have (2 channel, 3 channel, 4 channel. I know this is a non ABS car), difference in size of tires from front to rear. This is one area I'd recommend having an experienced track driver get the initial setup done. Once you've played with a brake bias controller a bit you'll start to get a feeling for how certain changes affect the balance under braking. Repeated attempts to lock up the brakes isn't necessary though.

I hope that made some sense, haha I can get rambling about this sort of stuff.

[thetalonguy]

Blue96GTS, curious why are you changing from Wilwoods to Stoptechs?

Luis, since you've had this setup for over a year, what is your opinion now? I've been doing some research on brake bias on changing Gen2 ABS front and rears. I'm more concerned on reaction to change in a road race track situation. I was thinking of doing a full caliper/rotor upgrade on the fronts (looking at various options) and either keeping the rears stock or putting my fronts in the rear. The last thing I want is screw up the ABS system while doing 150MPH. When I'm ready to upgrade, I was going to reach out to a few vendors for more input.

Personally, it doesn't have to be the balls out best solution, just a bit better than stock.

[Luisv]

Blue96GTS, curious why are you changing from Wilwoods to Stoptechs?

Luis, since you've had this setup for over a year, what is your opinion now? I've been doing some research on brake bias on changing Gen2 ABS front and rears. I'm more concerned on reaction to change in a road race track situation. I was thinking of doing a full caliper/rotor upgrade on the fronts (looking at various options) and either keeping the rears stock or putting my fronts in the rear. The last thing I want is screw up the ABS system while doing 150MPH. When I'm ready to upgrade, I was going to reach out to a few vendors for more input.

Personally, it doesn't have to be the balls out best solution, just a bit better than stock.

I've had the upgraded setup for about a year and a half and the car has been on two club mountain road cruises. When we head up for those we drive the cars in two groups. Those that drive hard and those that are going for the scenery. I am usually set to lead the hard core group. I drive those roads very hard and am quite hard on the brakes. I've loved the setup so far. Braking has been outstanding and a drastic improvement from the stock setup. I have yet to get any fade or overheating issues at all.

During regular driving the ABS has only been activated twice because of road incidents since the upgrade. Avoidance of an idiot that crossed two lanes to make a right turn cutting in front of me and a second time to avoid a dog that ran onto the street. In both cases I was going about 50 MPH and in both cases the car remained completely under control. I don't feel that the ABS was adversely effected at all with the upgrade.

As for track details... While I have driven a number of cars on road courses (spec racing and even an old open wheeler many moons ago) I can't tell you I have tracked this car because it is a secondary car for me. It is actually driven regularly so I can't risk tracking the car at this stage. The most aggressive test braking I have done with the car is at the drag strip. I have taken the car a number of times to the drag strip though and I have taken advantage of the track space there to get on the brakes hard from well over 100 MPH and, so far, not a hiccup. However, to be fair, I have not put track tires (HUGE difference) on there and hit a road course so I cannot give you a specific opinion there.

When I was searching around when doing my car, I did a good amount of research, i feel. What I found was that the setup I have on the car, with the exception of the rear rotors being slightly smaller, is the same setup that came on the Gen 3 & 4 cars. 4 piston brakes all around, piston bore sizes, front rotor size, etc. As far as I have been able to find, the ABS system is virtually the same if not exactly the same as the Gen 3 (at least) cars. In short, as far as I have been able to find, the upgrade I did was the same thing Dodge did when they upgraded the brakes on the Gen 3 cars.

The biggest complaint about the Gen 2 brakes I can find out there is usually fade. When you are looking at the setup the rears (look at my upgrade thread to see the difference) have very small brakes for what amounts to about 26" of rubber on the pavement. Yes, most of the braking is done with the front tires, but there is still work done with the rears. The stock setup has a great deal of front bias which is apparent if you just look at the swept area difference. It is HUGE. Doing this puts more strain on the fronts. Moving to a larger rear brake changes the front/rear bias putting more work in the rear. Our cars do have a very good weight distribution so we should be closer in a front/rear bias. Going from let's say a 70/30 bias (what I assume is the case, I've never tested it) to a 60/40 bias will dramatically reduce the stress on the fronts and reduce fade. When racing, if I had front heat issues at a track and/or a push on turn in, we adjusted brake bias to the rear. The concept is the same.

Ultimately, I would tell you to check with those that have done brake swaps to similar setups and see what, if anything is done with the ABS. I would venture to say that they run the ABS without incident. I say this because none of the kits out there (Brembo, Stoptech, etc) ever call for the removal or replacement of the ABS block/system. As a result, I would assume you are golden. I would tell you though.... I personally have not tracked the car to give you a firm answer.

[Blue96GTS]

Thanks for the comments everyone, great input. I'll keep the proportioning valve and do some deliberate testing before I get on the track at speed. Luis, know that I'll have your reference posts handy while I'm working on the brakes!

Blue96GTS, curious why are you changing from Wilwoods to Stoptechs?

The current Wilwood setup I have is "somewhat adequate" only, and was installed at some point by the previous owner. Likely good stuff for the time. The calipers (old Wilwood Superlites) are discontinued from a Wilwood perspective, and were recommended to be replaced (by Wilwood). I successfully rebuilt the calipers 2 years ago, and they have worked OK for me the last 2 track seasons. Choices of pads for these discontinued brakes are few - only Wilwood pads for the most part. Wilwood rotors are a 'disposable' item, not to be turned by your local NAPA, only replaced - these rotors have seen some hard miles... The rotors are a paltry 12", and look a bit smallish under my newer Gen5 wheels... Swept area is very tiny compared to any newer Viper with newer brakes. This year alone I went thru 3 sets of front pads (Wilwood BP-20), before switching to a more aggressive race pad. This certainly helped my pad wear problem, but also seemed to add more heat to the rotors, resulting in noticeable brake fade after about 15 minutes of hard driving (600 degree Wilwood DOT-3 fluid, and the NASCAR style recirculation setup notwithstanding). I feel I need a bigger heat-sink! After much deliberation of whether to go to a Gen3/4 setup, Brembos, Baer or StopTech - I settled on the StopTech BBK, 4 pot. My impression of reading about the StopTechs on the various forums was such: a.) ease of install b.) large availability of pads, c.) people seem happy with their setup. I'm not a 10/10ths driver, I do enjoy getting the beast out on the track, pretending my best impression of Ricky Bobby... I don't completely "trust" my existing brakes, which is not a good feeling while on the track. HTH


Although there is no way I'll compete with Luis' posts, I'll post some pics of my progress along the way once I get into the project.

Thanks again for the input.

[thetalonguy]

Thanks for the info Blue. Now I can definitely see why you're looking to change them out.

I'm looking to upgrade before the start of next race season. I pretty much started tracking my car this year and haven't had too much brake fade with aggressive street pads and stoptech slotted rotors and Motul. At my last track event, I had this new Z06 in front of me who couldn't drive; he wouldn't wave me for a pass. At one point, I was on the straight away and he jammed on his brakes way too early. Then I had to jam on mine and the pedal pretty much went to the floor and I was inches within rear ending him. That's when I lost confidence in my setup. I'm not a 10/10ths guy either since I drive to and from the track; but there is definitely more to be desired. I must say that the ABS is nice to have. :-)

[thetalonguy]

Thanks Luis, I greatly appreciate your input. Makes me feel a bit better. Thanks for your G3 upgrade DIY as well. I've read that a few times through to gauge a sense of what's involved.

[Blue96GTS]

In regard to your brake fade, perhaps it's possible you were suffering from knockback? Do you like to "use the whole track" including the rumble strips? Perhaps were both just outdriving our vehicles, and should both immediately upgrade to Gen 5 rides...

As for your near-rear-end adventures, I can appreciate the pucker factor you must have felt. Here's a short clip of me almost accidentally purchasing the bimmer I was trailing early this season. Felt a bit closer than it looks. You can also briefly see a slow corvette I passed along the way. Perhaps it was your guy!

[thetalonguy]

In regard to your brake fade, perhaps it's possible you were suffering from knockback? Do you like to "use the whole track" including the rumble strips? Perhaps were both just outdriving our vehicles, and should both immediately upgrade to Gen 5 rides...

As for your near-rear-end adventures, I can appreciate the pucker factor you must have felt. Here's a short clip of me almost accidentally purchasing the bimmer I was trailing early this season. Felt a bit closer than it looks. You can also briefly see a slow corvette I passed along the way. Perhaps it was your guy!

Sure, I'll take a GenV for track duties; just never getting rid of this one :-)

Yea, you were coming in hot and heavy on that one.

You want to see pucker?

Fast forward to 14 min mark. This is the same day I was just talking about. I went out first on this session to avoid traffic; lapped the Boss Mustang and wanted to hunt for more.

I was redlining 4th gear at about the finish line. Was going pretty fast so I held on to the brakes too long, but tried to steer as well. Result was 180 about 2 feet from the guard rail. My passenger was a local VOA member with an ACR. It was my first time at that track, so he jumped in for some guidance. Gave him a good jumble.

https://www.youtube.com/watch?v=9p0_UNFXfhs

[Blue96GTS]

Nice job on not wadding up your car. Holy buckets. I can see how that corner might give some fits. Looks off camber at the top. Your line looked good and things seemed smooth. Well, up to that point anyway! Seems like once they go, it feels hard to do anything other than hang on. I've had a couple of accidental 180's myself and wonder what someone with more talent than myself would have done at that split second of "oh-shit". Assuming that guard rail wasn't an issue, do you think you could have corrected your way out of things? What was your mistake?

[thetalonguy]

Assuming that guard rail wasn't an issue, do you think you could have corrected your way out of things? What was your mistake?

Well I knew my mistake was that I was asking too much from my tires at that point. I was braking hard and trying to steer into the turn at the same time. Couple that with that turn that has a quick elevation change which left me with even less available traction.

I knew it was going to happen too...a split second before I lost it, I said crap, you held the brakes too long, clench up and hope you don't crash.

I'm not sure what I would have done differently after going into the spin. However, I was youtubing that track a couple days before the event and I watched this one video with a guy driving an older M3 who crashed on the same turn. He lost traction, but tried to fight it by yanking the wheel back and forth. He eventually crashed about 10 yards further from where I was. So when I went into the spin, I just put both feet down, held the wheel and accepted my lucky fate.

[CobraMarty]

I have done brake upgrades using Chrysler parts. I have not done Viper brakes but I have done Jeep brakes and have used the Viper Master Cylinder.
I know that Jeep is not a Viper but I have made BBKs for the Jeep Cherokee and used Jeep Grand Cherokee SRT-8 Brembo calipers and Mustang Shelby 14" and 15" rotors. I have used the front 4 and 6 piston Brembo calipers and the rear 4 piston Brambo calipers. Chrysler uses the same Brembos across all their lines.
HERE- Jeep Big Brake Kit

You went from 42/38mm to 44/40mm on the front and on the rear went from single 42mm piston to dual piston 42/38mm. You need more volume. This is why you have increased pedal travel. The spongy pedal might be air in the lines as bleeding with the ABS is more difficult. I use speed bleeders which has made bleeding easier.
I have had to upgrade my Master Cylinders to larger bores for improved pedal feel and travel.
Here is a comparison of Master Cylinders that I have used- RockAuto.com is a great place to find out information about all this.

Master Cylinders for Jeep XJ/TJ-
1" bore, Stock XJ, F-12x1BBL, R-10x1BBL, rod 2.12" Centric #130.67018('98 Cherokee)
1.0625" bore, some '98 ?'99-03 Durangos, hit or miss, *must check*, Centric 130.67024 same fittings
1.125" bore, '03 Durango, Viper, Cardone #132967, Dorman #M630162, same fittings
1.125" bore, '03-'10 Viper, '99-'04 Dakota, '99-'01 Durango, Centric #130.67025 premium, #131.67025 standard ?different reservoir
1.250" bore, Dodge '99-'03 1500, 2500, 3500 pickup, Centric #130.67023, same fittings, pressure rod 1.09"(will need to lenghten rod)

You might want to try the 1.250" bore MC. It will give you more volume which you need for the front and for the rear.

[Luisv]

I have done brake upgrades using Chrysler parts. I have not done Viper brakes but I have done Jeep brakes and have used the Viper Master Cylinder.
I know that Jeep is not a Viper but I have made BBKs for the Jeep Cherokee and used Jeep Grand Cherokee SRT-8 Brembo calipers and Mustang Shelby 14" and 15" rotors. I have used the front 4 and 6 piston Brembo calipers and the rear 4 piston Brambo calipers. Chrysler uses the same Brembos across all their lines.
HERE- Jeep Big Brake Kit

You went from 42/38mm to 44/40mm on the front and on the rear went from single 42mm piston to dual piston 42/38mm. You need more volume. This is why you have increased pedal travel. The spongy pedal might be air in the lines as bleeding with the ABS is more difficult. I use speed bleeders which has made bleeding easier.
I have had to upgrade my Master Cylinders to larger bores for improved pedal feel and travel.
Here is a comparison of Master Cylinders that I have used- RockAuto.com is a great place to find out information about all this.

Master Cylinders for Jeep XJ/TJ-
1" bore, Stock XJ, F-12x1BBL, R-10x1BBL, rod 2.12" Centric #130.67018('98 Cherokee)
1.0625" bore, some '98 ?'99-03 Durangos, hit or miss, *must check*, Centric 130.67024 same fittings
1.125" bore, '03 Durango, Viper, Cardone #132967, Dorman #M630162, same fittings
1.125" bore, '03-'10 Viper, '99-'04 Dakota, '99-'01 Durango, Centric #130.67025 premium, #131.67025 standard ?different reservoir
1.250" bore, Dodge '99-'03 1500, 2500, 3500 pickup, Centric #130.67023, same fittings, pressure rod 1.09"(will need to lenghten rod)

You might want to try the 1.250" bore MC. It will give you more volume which you need for the front and for the rear.

The thread was originally posted back in 2014 when I first did the upgrade. I don't know where I had replied it but I have since decided to keep the original master.

I would like to mention though that I respectfully disagree a bit with the larger bore on the master after having driven the car. While there is more volume in the pistons it is not enough to warrant a larger bore. In the original upgrade thread I mentioned the spec on the Gen 2 brakes and Gen 3/4 brakes. In both cars the bore on the master is 1.125" despite the larger volume in the calipers. The Gen 3/4 uses the same piston setup as what I went to.

In the end (again, don't recall which thread I replied to) what I found was that the pedal travel on the car is only marginally longer. The tiny bit of extra travel I got was nice and I found that it allows me to better modulate braking. The original setup was way too short. To the point that it was almost no travel at all. The other thing that made me keep the original master is that by going to a larger bore you decrease pad pressure. The bore size is inversely proportional to the pressure at the caliper. Larger bores require more pressure at the pedal to deliver the same pressure at the caliper.

Bottom line, I anybody would ask me, I would definitely recommend that they keep the 1.125" master if you are doing this upgrade to the Gen 3/4 combo.