I have the same problem with my 86 track car, pedal was good until I put Big Blacks 928GTS with 94 C2 Turbo rotors. I have replaced every thing from a 89 Turbo S brake booster, pedal cluster, push rod and set to proper length even put a adjustable brake bias in didn't help. Talk to Charly at Wrightwood Racing he siad when you increase caliper piston side you need to increase the master cylinder size and the problem will go away. The calipers need more volume of fluid and smaller master has to travel farther.

 

"I have bled the brakes on this car well over 4 or 5 times using a power bleeder and cannot get the pedal to stiffen up. Pedal feels fine with the system pressurized with the bleeder. Its really a pain. The point i am at now is the first application of the pedal feels spongy and sinks a little lower than what is normal, however pumping it a second time and it feels perfect. "

This seems to encapsulate the exact symptoms of mine and quite a few others that I've spoken to. Very hard to put a finger on. I thought it might be due to RHS but obviously not. ABS? Not 100%. It could be the m/c but then I had someone describe how putting larger m/c in would increase the pedal travel so I'm kinda hoping the drilled rotors will help me. They feel better on the street but this means nothing. Could be more hopefull than anything. I'll see tomorrow.
Oh, and I also started looking at other Porsche models for m/c options but haven't got anything that looks like a straight swap. Apparently the one to go for is from a 1990's 7 Series BMW but that's the one I bought from Ebay and I sent a pic to the seller of ours and he said they didn't really look interchangeable, but I bought it anyway. If I have no joy with the drilled rotors I will look at this again.

 

it would make sense that by adding larger piston cylinders in the front caliper upgrade that a larger m/c piston would be helpful

 

Its possible the master cylinder we used is the issue as i dont remember the specifics of it. But i am pretty sure he specifically bought it because it had a larger piston size, though i am not sure by how much. I was actually skeptical of the master cylinder and thought it was the cause of the spongy pedal, i guess its good to know we are not the only ones having trouble with this. Its very unnevering when i take the car for test rides around the block and have the pedal go that far to the floor, i can only imagine how he feels out on the track!

My other guess is there was still air in the ABS pump, however it seems this problem is not exclusive to ABS equipped cars. I know some new cars nowdays require the ABS pump to be put into a specific 'bleed' mode when changing brake fluid to get all the air out, others require 30+ psi to be applied to the brake system. I never exceeded 15-16psi when bleeding my friends cars.. i didnt want my Motive bleeder to explode .

 

I ordered a 1993 928GTS master cylinder 928-355-011-24 will see how well it fits.

 

Does the car have to move first before the pedal softens up? i.e. if the brakes are firm stopped in the pits... and if you wait a little while, and they're still firm in the pits, but as soon as you start driving, do they get soft then?

If so, then your rotors are pushing the pads (pistons) back into the calipers too much. The "soft" part of the pedal travel is moving the pads back to contact the rotors... Why THAT would be happening is an entirely different issue.

 

Van, what your referring to is pad knockback. The pedal on my friends car is soft basically all the time, not moving, straight line, turning..etc.

Ive never experienced pad knockback on a 944 series before, my friends '04 WRX gets it very bad. I guess its just the differences in hub, rotor hat design.

 

ATE Specs for the 928GTS master cylinder. Same size cylinders as the 944T with ABS. The 944T non-ABS has a longer cyclinder.

Driver Position For left-hand drive vehicles
Trade Numbers: 010020
Diameter 1/ Diameter 2 [mm] 23,81/20,64
Number of ports 2
Thread Size M10x1
Length 1 [mm] 17,6
Length 2 [mm] 159

The 944T spec for reference.

Master Cylinder, Non ABS
Trade Numbers: 010025
Braking / Drive Dynamics: for vehicles without ABS, Ø1/ Ø2: 23,81/20,64 mm,
Number of ports: 3,
Thread Size: M10x1,
Length 1: 21,1 mm,
Length 2: 159,8 mm

944T with ABS
Braking / Drive Dynamics For vehicles with ABS
Trade Numbers: 010024
Diameter 1/ Diameter 2 [mm] 23,81/20,64
Number of ports 3
Thread Size M10x1
Length 1 [mm] 17,6
Length 2 [mm] 159


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Put me down for squishy pedal feel. I have turbo S brakes and have bleed a number of times. I might try a new master since mine is stock with 225k miles now.

 

That's right. I've only felt it when a wheel bearing has started to go south...

It's interesting that several people with the big reds are experiencing the same phenomena - it made me wonder if somehow the 993 brake rotor doesn't sit squarely on the hub...

 

I have never liked the way mine feel either. I can't recall whether it was that way before the big blacks. Since mine is an 89, it already had the larger master cylinder. I believe the pistons are identical in the 89 and the blacks, so fluid volume shouldn't be the problem. They work great, even with the ABS turned off, but they feel lousy.

West

 

One thing I'm picking up here is most have used a power bleeder. My 86 has the stock MC, 5/33 bias valve, ATE blue, and big reds with good pedal pressure.

When I first intalled them they were mushy and it would lock-up too easily. I rebled them (motive bleeder) three or four times to no avail. On the last bleed, with the bleeder pressurized and the valves open, I pumped the pedal through the full range of motion a few times and the pedal stiffened up to the point where it's pretty close to stock.

Now if I can only stop it from squealing under light-moderate pedal pressure...........

 

Master cylinder didn't change for the 944T S. The master cylinder is the same P/N from 87-91.

http://www.paragon-products.com/Brak...355.011.01.htm

 

why is this thread telling me of issues with big reds 1 day after i just bought the whole setup

 

Same here. At first it didn't squeal at all. Then horrible squealing. Then had car aligned. Squeal went away. Now its back, but only at light-moderate pedal pressure.