Todd heres an idea, can you adjust your rear brake bias to complete soft and drive the car to see if there is a difference. That maybe one way to confirm your problem is in the rear.

Actually thought about that. I just don't see C6 hubs having excessive deflection putting around town. I can feel no play in the front bearings. The napa number is 88128 but it's a roller.

They shouldn't, and if they did, you'd feel it with the car up in the air. Mine only do it on a very hard corner, but I can still manage it on the street.

One way to tell what the heck is going on is to find a parking lot large enough for you to do a "skidpad test". Run the car hard enough to push the pads back, then coast to a stop (still turning in a circle). Jack the car up and you'll see the pad that is pushed away from the rotor. It's a very small gap, but it will be obvious. Trouble is finding a location to do this.

Nice. That's the attitude that makes me happy to help people out.

Thanks Matt. I should verify it first. I'm pretty sure I used the parking brake since it's internal on Wilwood to come to a stop and jacked up both ends. I seem to remember the front pads having drag and the rears not.... That's what got me focused out back. I'm ordering the updated bearings out back. I have a suspicion of what may be going on here. Hell I can drive it around the block and wiggle the car around and the knockback will be there. I'd bet a large sum it's out back still....

I really hate to point the blame at Wilwood. They have also been good to me. The whole being the guinea pig for both companies and having all that axle end play just pissed me off. I'll figure it out....on my dime. LOL

I have chased this problem for years...

List of what I have learned

• Roller bearing on a Ford 9 inch won't work for road racing.
  
  GM went to a stiffer front wheel bearing on the ZR1, STS-V and CTS-V to fix brake knock back.
  
  The bigger the rotors the bigger the problem.
  
  2 psi valves won't do it
  

The test I use is to do a figure eight in a parking lot at max lat then measure the brake pedel travel.

If you can't fix the problem learn how to left foot brake on the straights to pump up the brakes. I have done this for years. :yes:

Mark

Thanks Mark....they aren't a problem on the road course. One quick pump down the straight or between corners and they work great. The autocross is the real problem! 15-20 turns in 45 seconds. :yes: I do have the roller bearing and pulled them to reinspect last night and they feel like new. I have decided to try a 10 psi residual valve. Bret at Air Ride is using one on the rear of velocity with good results. If that doesn't prove to be reliable and make me happy then it's on to a tapered bearing and probably hydraboost.

Hi Mark,

Do you know the GM part number for the stiffer ZR1 front wheel bearing?
I want to make sure I have the right ones...

Let me see if I can help here..

1st I agree with Mark on the roller bearing, they are not the best for road racing.. tapper is the way to go..

2nd I would see about rotor run out on all 4 rotors, when setting up a car for racing I always have the rotors turned on a brake lathe.. when you have rotors that are assemble to hat, hub, & rotor you are going to have a stack up tolarance of pcs that have been machined probably flat & parallel within .002 to .004 & you added it up & you have an excessive amount of rotor run out which will knock back the pistons & have pulsation in your brake pedal.. run an indicator on the faces of your axles & hubs & see what your run out is..

3rd Caliper flexing on brackets.. The Wilwood parking brake assembly/caliper mount has some flex on it.. remove your pads & grab the caliper with your hands & you can move it.. For racing I make new brackets out of steel or aluminum with gussets machined into them to make them stiffer & it makes a difference on how the pedal feels & when you are going into corner.. look @ it this way.. large diameter rotors, hydraboost, high friction brake pads & racing .. got to have good stiff caliper mounts.. attached is a rear mounting caliper bracket I made for racing.. for asphalt racing steel brackets are welded on rear axle tubes & front brackets are welded on a fabricated spindle/upright also have floating rear ends with large bearings.. Take into consideration that in some forms of racing they want piston knock back to free up rotational weight.. can be done with o-rings in calipers instead of square cut rings

I'm starting with some tapered bearings. Moser should be sending them out today. Thanks for the info on the rear brackets being flimsy. I'll definitely look into that if I'm unhappy after my new tapered bearings. :thumbsup: