KaiB

Man am I glad I didn't say that!!!

Veloce Raptor

Must have really hurt Bill Gates before he was rich when he said "I have a company called MicroSoft"...

Steve113

Its either:
Wheel bearing
Caliper sticking (needs rebuilding)
Rotor moving due to ?
Pad to large for caliper
Bad master cylinder or wrong size one
something clogging a brake line

Can be very tough to diagnose. Go through each wheel then look at overall system.


Driving style will not do it. As said before its mechanical

Deadeye

Just one data point. I had significant knock back and started with new rotors, pads and wheel bearing adjustment. No joy. I rebuilt the rear calipers because the dust boots were cooked (no leaks at the psitons) and the problem was cured. I theorize that the old seals were allowing the pads to retract further. No other changes and no more knock back

analogmike

As I mentioned, it's the curbing, whenever I hit the curb in the middle of no name straight at Lime Rock, I have knockback in the uphill. Don't hit it, no knockback.

Cory M

I think anything that deflects the suspension significantly can contribute to knock back: whether it's curbing, banked turns, high G turns, elevation changes, or bumps. On this particular track I had knock back whether or not I used the curbs, but every lap had tight turns, elevation, and some banking. I'm running much bigger and stickier tires than the original suspension parts were ever designed for, the bigger rotors accentuate the knock back further.

Here's a decent explanation of pad knock back:

http://stoptech.com/technical-suppor.../pad-knockback

I am going to rebuild the calipers and try out the anti-knockback springs. Seems like the only thing I haven't tried yet. I'm certain I'd go faster if I had more confidence in a solid brake pedal.

Cory M

A blown engine killed my 2013 season so it has taken a while to update this thread:

I rebuilt the calipers (front and rear) with all new pistons and seals and installed the anti-knockback springs from HRP. My knock-back problems are solved now and the pedal is nice and firm. When I rebuilt the calipers it was really tough to extract some of the pistons. Although the seals and pistons appeared to be fine visually and showed no signs of overheating or wear, they were really stuck together. I think the problem was that the parts sat on the shelf for a few years and things just got sticky since they weren't being "exercised" regularly. I didn't think they needed a rebuild because they looked fine but I was wrong. I guess that's my punishment for being a cheap bastard. I did the fronts first and saw a big improvement, then rebuilding the rears fixed it 100%.

I'm not sure if the anti-knockback springs actually do anything. The spring rate is a small fraction of the force it takes to move the pistons so it's hard for me to believe they helped much. That said they are pretty cheap (~$30 total) so I think it's worth doing.

I never really solved the mystery as to why the other driver didn't have any issues and I did but I'll chalk it up to him being unfamiliar with my car and being more cautious in the braking zones.

Thanks for all of the feedback!

paradocs98

Thanks for following up.

JasonAndreas

Hasn't Brembo somewhat recently (within the past two years or so?) redesigned their pressure seals to prevent/reduce knock back?

Cory M

Yes, saw those at HRP recently, unfortunately they didn't start selling them until after I rebuilt mine. I would be interested to hear what they changed about the seal to reduce knockback.