griffith500

Car: 1990 C2 with 993 Big Reds in front, 4-piston C2 in the rear, PV removed

I had a complete brake failure last weekend at the track: I have a firm pedal when the car is parked but it goes to the floor second time I hit the brakes at speed, with very little braking. We completely bled the brakes 3 times using a pressure bleeder but could not fix the problem - opinion was that it must be the master cylinder.

The rear calipers were just rebuild and I plan to also rebuild the fronts and replace the MC; I could get my hands on a used stock C2 or alternatively a C4/Turbo version.

Will the C4/Turbo version fit my setup without modification?

Would this be an improvement over what I have right now ( I am assuming I have a stock MC )?

Thank you in advance for any thoughts and advice.

993PET

You'll fall over when you see the cost of a new 964 Master Cylinder ovef $1k. I went to the 993 MC and booster with big reds on the front and 93' stock brakes still on the rear. Much better in every respect. BTW the C4/turbo has a different booster than the stock C2.

JF

KaiB

The C2 uses a vacuum booster whereas the C4 and T use electro/hydraulic - not the kind of swap you want to get into.

993PET has your solution; you'll want the 993M/C and its booster - search for fitting differences and parts to make all fit.

KaiB

Although for the life of me I cannot figgr this one out.

You say the brakes seem to funciton fine when the car is "parked"...is this with engine running?

Then they function fine the first time at "speed"...do they work at "non-speed"?

Then they quit altogether?

griffith500

Wow..I didn't expect this much of a difference in price between 964 and 993

I had no plans to migrate to the hydraulic booster and hoped I can simply use the bigger MC, assuming al the fittings / connection line up.

Thanks for the quick reply,
Tom

griffith500

During a hot lap, the brakes got very soft but the pedal didn't go all the way down to the floor. After bleeding all 4 corners, the brake pedal felt firm while at a standstill ( engine off and on - no difference ). Went out on track, decelerated from 100mph to 60mph and everything felt as usual...back to 100mph and repeated above but pedal went almost to floor, with close to no breaking. Repeated brake bleeding ( used 4 bottles of Motul in total ), went out again and got the same result. After coming in the pits, brake pedal felt firm again - at this point I decided to stop the bleeding and packed up.

Thanks again for your reply,
Tom

Blu RS

Sorry, but I've lost two long efforts to explain this but they fall off the system. So this will be short. The problem is caused by your rebuilt rear calipers. Their brake caliper seals are not letting the pistons retract. So the brakes get too hot, then the fluid boils and your pedal gets spongy etc..

I'm sure you could have detected that your rear brakes were getting too hot, or your rear rotors showed unusual heat discoloration, or you smelled more brake pad smoke than normal.

What happened was that your brake seals were either swollen from contamination from a petroleum product, or the seals were installed incorrectly or too much silicon brake paste was left residual underneath the seals in the seal ring lands. So basically the failure of the seals to allow the pistons to retract causes all the issues you've been confronted with.

The solution is to rebuild the calipers with new seals or possibly (this is a calculated long shot) to do a repetitive manipulation of the pistons up and down in their bores to try to get the offending excess silicon paste squeezed out from under the seals, if such may have been the case.

If you do end up removing the pistons to do a rebuild then make sure you have the seal ring lands fully cleared out of crud and excess silicon brake paste and only use brake fluid as a lubricant on the seals when you reinstall them. Of course you may continue to use silicon brake paste as a piston lube but just use brake fluid on the seals and the seal ring lands.

KaiB

I also have no doubt now that the problem is fluid related - and the above makes perfect sense.

Thank goodness - in many ways, huh.

griffith500

Good point and will check that out first thing after work today.
Thanks,
Thomas

Bill Verburg

2 points
1) hopefully you have the 4 piston rears w/ larger pistons, stock 963 is 28/30, 964RS & 993 is 30/34, you want the 30/34 as the system s very front biased even w/ the 30/34. Cups used 30/34 and removed the p/v too

2) no matter what, w/ either big red or just the 964RS front calipers and 30/34 rears you want the 993 23.8mm m/c(or 25.4mm RS). The 993 stuff is a bolt in the 964RS/C4 is not

KaiB

Bill, will the 25.4mm RS M/C function properly with the vacuum booster from the 993 in a 964 w/ big blacks fnt and 993s rr.

t, Kai

Bill Verburg

No, the whole system would need to be t/d'ed, a non-trivial task

KaiB

Gottchya, so the 23.8 it is then.

ejabour

There was a write up in Grassroots recently and when a 964 owner balked at the price of a new mc. He took his failing mc to a machine shop and had it bored and and sleeved for under $100.

griffith500

Thanks Bill for the useful information.

1) I do have the 4-piston calipers in the rear and we removed the p/v, too
2) If I understand your note below correctly, I can bolt in a 993 C2 master brake cylinder and bolt it in, right in place of the 964. Is this a correct assumption?

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Tom