1" or 1 1/8 wilwood master for Baer track kit?

[Zedzag]

I have the Baer Track kit 13" front and 12" rear brakes on my 81z. They are basically C4 vette brakes I was told. I can get a good deal on Wilwood master cylinders but I don't know what bore size I need. Do I need 1" or 1 1/8" or is that just preference. I want to track/auto-x this car and want the brakes to grip without killing my leg.The smaller the bore the more leg pressure it takes? What is the c4 vette master bore size? I asked Baer and they said "use a wilwood master".

[killer69]

i would start wit a 1.125 and see how it feel to you, that seems to be the biggest factor how you like it to feel , weather your usinf a single or dual diaphram booster what the vacuum is like or if your going manual???

another option would be to go get a cheep aftermarket one put it on try it and then buy the more expensive Wilwood one

[Zedzag]

This will be a power setup using the fctory 81 booster. There should be plenty of vacumm as my cam is only 228/232 @ .050 in a fuel injected LQ4.

[MuscleRodz]

Smaller bore, more pressure, but only too a point where it becomes a negative. 1" might make it twitchy, but that will more depend on the pads you use. I would do as Blake mentioned and figure it out with a cheap master before buying the Wilwood. FYI Wilwood 1" is 260-8555 and 1-1/8" is 260-8556

[Zedzag]

I guess I'll try the cheap tester master first. Where can I find a cheap 1" master with the outlets on the fender side? 1988 S-10?

[MuscleRodz]

any autopart store should have them, being canada, not sure how close you are to one though

[Apogee]

The 1988-1996 C4 Corvettes used several diffferent master cylinders over the various years, however most were 7/8" or 15/16" on a 8.75" or 9.75" single diaphragm booster. I would not recommend running a 1-1/8" bore MC with such small calipers as it will not generate adequate pressure without excessive leg force. Your factory 1981 Camaro Z-28 power brake booster should be slightly larger at 10.75", but I would probably recommend a 15/16" bore MC for your application given the information you've provided.

Tobin
KORE3

[CarlC]

Tobin is right on the money. An 1-1/8" will be ineffective. A 15/16" is a good starting point. I run a stock 1999 Cobra Mustang HB and MC, so if you can find the piston size it too would be a good starting point.

Here's something that may also be helpful http://www.geocities.com/torkerscamaro/torker.html

[Zedzag]

any autopart store should have them, being canada, not sure how close you are to one though
Not far once I give the sled dogs some hot sauce in their chow.

[Apogee]

FWIW, the front C4 calipers have twin-38mm pistons and the rear has a single 45mm piston. That equates to ~3.5 square inches of piston area per caliper up front and ~2.5 square inches in the rear.

For the sake of comparison, a C5 front caliper has about 4.0 square inches of piston area, 15% more than the C4 calipers, while the rears are the same. Since a C5 'vette runs a 1" bore MC with a dual-9" diaphragm booster, you should definitely end up with a smaller bore size. All of this is without considering pedal ratios, but they should be similar enough between the applications as to not tweak the results one way or the other too much.

I may not want to know the answer, but have you measured the amount of vacuum your engine generates at idle?

Tobin
KORE3

[Zedzag]

Tobin is right on the money. An 1-1/8" will be ineffective. A 15/16" is a good starting point. I run a stock 1999 Cobra Mustang HB and MC, so if you can find the piston size it too would be a good starting point.

Here's something that may also be helpful http://www.geocities.com/torkerscamaro/torker.htmlI'll look into the s-10 master in that write up and see how they work.
Thanks.

[Apogee]

CPP sells a C3 Corvette type MC with a 15/16 bore for $59 that isn't a bad deal for a new MC. The mid-80's S10 MC will require machining to make it fit and has a somewhat leak prone cap in my experience.

Tobin
KORE3

[tommycomfort]

I asked the same question about my track system to the Baer brake guy at a goodguys event. He said that they (Baer) use the 15/16" master now and even told me I could use one from a 1977 Malibu Classic and get it at the local parts store. Hope that helps.

[Zedzag]

I called Cpp about their 15/16 master and they said the 1" master would give a better pedal feel from their experience. Now I'm getting confused lol.

[Vegas69]

I'd listen to Tobin.....

[Zedzag]

Well so far Tobin, Baer and Mary Pozzi all say try the 15/16 bore so it's "winning" the race. I might call wilwood tomorrow and see what they say but I didn't see them offering a 15/16 bore.

Also I went to the local parts store ans the s-10 master is dual bore size 24mm and 36mm so I'm not sure how that would work.

[CarlC]

Here's another vote on top of my first one. I tried a 1" MC with a stock booster using a cam that made 12inHg at idle. With both legs I could barely get the tires to skid. It was not fun at all. This is with the same C4 PBR currently on the car.

[Apogee]

Carl's experience is pretty typical for guys running C4 PBR calipers with oversized master cylinders and/or marginal vacuum. Not to throw stones, but I suggested CPP for their 15/16" MC, not so much their phone tech support.

As for the S-10 MC you keep mentioning, there are two versions, one for power applications and one for manual. They are both stepped bore, quick take-up master cylinders designed to be used with the low-drag cast iron calipers that GM started using with various applications and only vary by the larger bore. The power units are 24mm/36mm like you said and the manual units are 24mm/31.75mm IIRC. The piston seals for the low-drag calipers were designed to retract the pistons further than the previous design...while good for fuel economy, it required additional volume from the MC in order for the pads to make contact with the rotors. Until the pads do make contact with the rotors, your system doesn't generate much in the way of pressure. Once the system becomes hydraulically locked or static is when you begin to generate pressure. The larger bore in a quick take-up master cylinder pushes a high volume of fluid until a certain pressure threshold is reached and the smaller piston takes over to generate pressure. It's like having two MC's in one.

Have you had a chance to measure your vacuum at idle?

Tobin
KORE3

[Zedzag]

Carl's experience is pretty typical for guys running C4 PBR calipers with oversized master cylinders and/or marginal vacuum. Not to throw stones, but I suggested CPP for their 15/16" MC, not so much their phone tech support.

As for the S-10 MC you keep mentioning, there are two versions, one for power applications and one for manual. They are both stepped bore, quick take-up master cylinders designed to be used with the low-drag cast iron calipers that GM started using with various applications and only vary by the larger bore. The power units are 24mm/36mm like you said and the manual units are 24mm/31.75mm IIRC. The piston seals for the low-drag calipers were designed to retract the pistons further than the previous design...while good for fuel economy, it required additional volume from the MC in order for the pads to make contact with the rotors. Until the pads do make contact with the rotors, your system doesn't generate much in the way of pressure. Once the system becomes hydraulically locked or static is when you begin to generate pressure. The larger bore in a quick take-up master cylinder pushes a high volume of fluid until a certain pressure threshold is reached and the smaller piston takes over to generate pressure. It's like having two MC's in one.

Have you had a chance to measure your vacuum at idle?

Tobin
KORE3I'll get a vacuum reading and go from there. I thought CPP would be able to share their experiences with C4 brakes and appropriate masters to get as many experienced based opinions. I'll try the 15/16 and go from there. I just want this car to stop without having to two foot it and without being way too sensitive.

[Apogee]

Short of hydro-boost, I think you'd have a hard time making a C4 set up feel overly sensitive. Good luck with whichever MC you choose and please post back with your results. I'm always curious to know how these turn out since there are as many different opinions on brake feel as there are people driving them it seems.

Tobin
KORE3

[Zedzag]

Thanks again for everyone's help. I'll post my results in the near future.

[Zedzag]

Well it’s been a long time for an update from me. Partially because the car sat for years, partially because I drove it for a while with crappy brakes and partially because once I solved the problem I just plain forgot. I tried two different 15/16 masters, a new booster, bled the brakes to death, all new friction and even rotors to eliminate any possibility of glazed pads etc and nothing helped at all. Then I put in a Detroit Speed booster/master combo which has a 1” master and the car stops really good with just a set of Raybestos cheap pads in there. Can’t wait to try some performance friction like Hawk etc. I should’ve tried this approach years ago.

[tommycomfort]

Well it’s been a long time for an update from me. Partially because the car sat for years, partially because I drove it for a while with crappy brakes and partially because once I solved the problem I just plain forgot. I tried two different 15/16 masters, a new booster, bled the brakes to death, all new friction and even rotors to eliminate any possibility of glazed pads etc and nothing helped at all. Then I put in a Detroit Speed booster/master combo which has a 1” master and the car stops really good with just a set of Raybestos cheap pads in there. Can’t wait to try some performance friction like Hawk etc. I should’ve tried this approach years ago.


Thanks for the update! Bottom line is that if you find it more drivable you're heading in the right direction.