kmagvette

I find myself doing this almost out of habit. The only time it is not necessary is when front and rear pads are new. I was trying to figure out why this is the case and this is the best I could come up with: When the pads are new, the pistons can't get knocked back too far.

Brakes are stock on a C5 Z06, new calipers (didn't help) SS pistons and lines. No leaks or any other issues with the brakes.

Has anyone come up with a way to mitigate this? I imagine reducing knock back is the challenge; is that possible? Are there other things that can be done to bring the pedal back up without the double tap?

Thanks for any suggestions.

wallyman424

near the end of straights or after a long string of turns without braking, i will tap the brake pedal with my left foot to build up pressure (while still on the gas of course).

varkwso

I learned this racing a 1972 911S a long time ago and it still applies to most calipers on most cars today. As he says - left foot tap to seat the brake pads works wonders (watch the GT3 Cup cars at most races...) and works like a charm on the C5

CHJ In Virginia

I have the same problem. I think that it comes from caliper spread due to heavy usage. Over time, the caliper mounting surface where it bolts to the bridge gradually twists and is not parallel with the fixed surface on the other side of the caliper that presses against the pad backing plate. The backbone of the caliper gradually flexes and the piston is also not parallel with the fixed surface. This requires more brake fluid movement to push everything together = low pedal and a lot of pad taper too. The only real fix is new calipers or upgrade to Wilwood, Stoptek or similar performance caliper systems. Wilwoods are sitting on my workbench waiting on time to install. I'm through messing with it.

Aardwolf

I do this as well. I have found the pedal travel to get long even with brand new calipers. I have wondered a few times if vacuum also plays a role, as pressing the pedal also puts more vaccum back in the booster doesn't it?

Bill Dearborn

I ran into this problem the first time I took my 97 to the track with 2700 miles on it. Its due to a combination of brake pad taper caused by the caliper twisting around its center as the brakes are applied and piston knock back as you go around turns and run down a long straight. If you look at your pads you will see they are tapered lengthwise. As the taper increases from hard use the pedal will drop to the point you will not be able to heel toe since the brake pedal will be below the gas pedal. Here is a picture of some pads from my car when I still ran stock calipers:



These pads are from the right side of the car. The pads on the left side taper the exact opposite of these.

What is interesting is the taper reduces if you use the pads on the street after the track session. The pedal will gradually raise as you put miles on the car.

You can reduce the on set of taper to some degree by changing the way you apply the brakes. When you first put your foot on the pedal do it easily and slowy depress the pedal. Once the brakes have engaged fully increase pressure as needed. This seems to extend the useful life of the pads by several track sessions.

Bill

Webz

I apologize for my ignorance, but what is piston knock back and why would occur in turns and on long straights?

varkwso

The rotor is mostly "true". The pads get taper and the calipers get spread. This causes the high points to get "knocked back" when the brakes are not engaged - this gives space before the pad reengages on the rotor with brake application (can also happen with bad wheel bearings). The pre tap takes away this space.

My 2002 ZO6 has a much better brake pedal "feel" then my 1999 FRC. In the FRC I have replaced all the calipers with new GM ones and new SS lines - it still has a crappy pedal - stops great - just feels bad doing it. We have thought about a new MC and booster since it does have ~160,000 miles on it now. Mostly weekend miles I might add...

02 ZO6 has 47,000 miles (many on track) still has original calipers.

986_C6

Great info!

Bill Dearborn

Looking at the picture I posted above you will note the caliper has to rotate counterclockwise with brake application to cause the wear shown. When brake pressure is released the caliper will rotate back to its normal position (this is a small movement). The lower inside pad is away from the rotor due to the rotation. Then as the rotor comes around it will brush the upper part of the inside pad and knock it toward the piston. Now you have increased piston travel at both the bottom and the top pistons. When you go around turns the bearing run out causes the rotor to brush the pad again and you get more push back. The next time you hit the brake pedal the pad has to travel this increased distance before it hits the rotor which results in increased pedal travel.

Your brake tap takes up this space.

Bill

jrz1

Another potential cause is your hubs loosening up. I recently replaced mine and when I did it nearly eliminated the pad knock back issue. I'd been driving on the stock hubs for 6-7 track events and I think they were just getting worn.

GAS

STOPTECH isn't the long term answer either. I have the six piston fronts and four rears and it is an issue for me also. Same solution as everyone else....as soon as I am in the straight I tap the pedal three to five times to get ready for the next corner. Shouldn't have to for what they cost but it is the only way I have found to have firm brakes at corner entry.

kmagvette

Wow, guess I'm not alone. I never considered that the caliper would rotate in that mode. Any idea what is yielding? Bracket, pins, caliper itself?

On the knock back issue I never considered bearing run out either. I am replacing my front hubs out of concern for flange integrity. Maybe that will help the knock back. I'll know in about a month.

I have just been giving the pedal a little primer tap with my right foot to bring the pedal up; I like the idea of using the left foot - it needs something to do in the straights other than watch the right foot have all the fun.

Look out Bill, I may be as fast as you now

Just some clown in a QS Z06 at The Glen.

the blur

I'm a double braker too. not so much out of piston knock back, but more out of life preservation.
and I find myself braking early too.
I wonder if the habit started out of extended pedal travel ???

either way it works for HPDE's, but it would never work for racing.

sometimes I do a confidence tap on certain turns... that I know I can do without braking at all. like that last left hander at WGI. a little tap sets the front end down, but I know it can be done without braking at all.

kentz06

I have the StopTech BBK kit F 6 pot & R 4 pot in my '07 Z06 and have the same issue. I installed the heavy duty hubs with no noticable change. My guess is the Hoosier R6 tires, Pfadt poly bushings, Pfadt competition sway bars, Moton Club Sport coil-overs and -2.8 degree camber are all flexing the other suspension components. The rotor can then knock-back the pads. IMHO this is not a brake system issue.

When instructing at the NCM Road America HPDE I had to left foot pump the brakes for 8 of the 14 corners. Andy Pilgrim drove my car for 3 hot laps and he was perfectly comfortable with the issue. Said he had to pump the Cady's brakes for every corner for 4 years!

I have used up the first set of front StopTech rotors and have new ones on order along with new hardware. There is a way to modify the hardware installation to make the rotor float approximately .010" to .015" which should help reduce the problem.

I have decided that I am driving my car like a race car without a full race specification suspension. I will try to minimize it but will continue to deal with it as required.

Dean

varkwso

Replacing hubs has not relieved the knock back issue on the FRC and all are new this year. The system has flex and even with a pedal not being perfect the brakes are great on these cars.

I wish I could afford new pads (and tires) every competition weekend.

AU N EGL

and to make sure you still HAVE brakes.

mwvettec5

throwit

I have severe pad taper, and calipers that are 8 years old and I NEVER double tap (knock on head ).

I have a friend with a P-Car who had this same issue and we thought for over a year it was a "air in line", "air in master/slave cylinder", "air somewhere?" issue, but it turned out to be the hub. Basically what was happening was that the hub was loose, or broken, and the rotor was being "wiggled" to a slight angle while driving. Once the brakes were applied the rotor was straightened up and then on the second tap everything felt solid. We did not have to replace calipers, pads, lines, fluid, anything...

Get the hubs checked/replaced.

69autoXr

No, pressing the brake pedal depletes the booster. Any part-throttle or idle of the engine restores the vacuum in the booster. You can actually deplete the booster of vacuum by tapping the brake while at WOT, and if you didn't come off the throttle to brake, you'd find a very hard pedal and wouldn't be able to slow down. Normally this isn't an issue though since we usually come off the throttle for the braking zone.