dizzyj

Looking for some more advice. I've posted about this before, but have not solved it.

My car has an issue where on the track, often the brake pedal catches really low to the floor unless I give it a maintenance pump.

Once I give it a maintenance pump, it will be stiff for a long time (sitting in the garage.etc), but on the track it will below again. Its as if the vibrations are either a)getting the exact same volume of air into the system, or b)pushing the pads further into the piston that it should.

I've been calling this my pad knock back issue, cause thats the closest thing that it seems like.

The car is an 89 turbo with s4 (med black) fronts and turbo rears (both factory) and has ABS.

I've replaces both hubs with raceware aluminum hubs with brand new bearings.
I've replaced the rotors.
I'm running motul 600 (and it will do it without the brakes warming up).
I use a power bleeder, and have bleed and re-bleed over and over including the extra port on the mc
I have ss brake lines


Things I'm thinking of trying next:
- rebuild the calipers (usually rebuild is for a sticking pad right?)
- new brake MC (although I read that if I was getting air into the MC, I would be able to slowly push the pedal to the floor.
- new brake booster? any chance this could cause issues?
- any way the clutch system could bleed air into the brake system? I thought they just used the same reservoir.

The other thing im thinking of is disconnecting the rear brakes, to see if I can isolate front/rear issues.

Any other things to try?

rlets

I read the other day that bad rear wheel bearings can cause knock-back issues.

Rich

eman930

Sounds like a Bad Master cylinder, One of the O-rings is bad and letting fluid slowly leak by and your loosing pedal pressure, Once you pump it up it builds pressure again, Highly doubt that its a Bad booster, If it was a booster youd have a vacuum/idle problem or be loosing fluid. The clutch does use the same reservior but it wont drain the brake resevior or get air into the brake system

dizzyj

k, i guess its off to buy a new mc

jerome951

Try this to verify pad knockback.
Just after the last turn on the longest straight, tap the brakes w/ your left foot to set the pads against the rotors. If you have a low pedal at the end of the straight, it's unlikely to be knockback and likely the MC (they aren't fun to replace).

I've found that if I run my wheel bearings on the loose side (recommended by Karl when using his billet hubs) I get pack knockback, but only after certain corners. I'm used to resetting the pads w/ my left foot so I don't have to lift off the gas.

The prolonged stiffness after a maintenance pump (gee, that sounds bad) means you probably don't have air in the system.

dizzyj

See, this sounds a lot like what is going on. and If I was getting air into the system, I would be surprised that the pedal catches at the exact same spot every time (just real low).

I'm replacing the mc anyways. I have the engine mostly dismantled, so I might as well. If it is pad knockback, I'm not sure what to do about it. everything on the front is new. I have not messed with the rear brakes yet. Like I said I thought about disconnecting the rear and plugging the mc, just to see if I still have the issue (isolating front from rear).

Its just real annoying, and im the only one I know with a 944 that has this issue.

eman930

Ive Never heard of pad knock back before, If your rotor/hub is moving that much to push the pads back, Id think that couldnt be to good for the bearing/hub assembley.

jerome951

I've met a few other 944 racers who have or have had this issue.

Highly unlikely the rear is causing the knockback considering how tightly they're torqued. If so, one of your rear bearings is toast and you'd probably hear it grumbling at you.

Then you start cornering hard with sticky tires, it puts a tremendous load through the hubs and wheel bearings. I've found if I have any noticable play in the front wheel bearings I'll get knockback in highly-loaded, rough turns (usually those where there's a dip or bump in the track during hard cornering or where I use the gators at trackout). I can tighten the wheel bearings enough to eliminate the knockback but that's probably too tight for the bearing.

People who don't have this issue just aren't driving hard enough.

dizzyj

What about pad knockback springs? any experience with them?

9turbo51

Don't know about the kick back but I have had less than great experience with power bleeding.
I recently power bleed my brakes after having system open. I had to go back and do manual bleed to get a large amount of air out.
Local race shops will not power bleed.

400hp944

Are you still running the "anti squeal" vibration dampeners in the calipers? If so, remove them. I had the exact same issue with my track car. I recently replaced the master and the rear bias valve (I was going through 5 sets of front pads to 1 rear). While I was doing this I decided to pull those vibration dampener things out as well. I'm not sure which part solved the problem, but it's gone!