Caliper/Brake Pad Knockback Remedies?

[A.T.]

Hey Folks

A while back while steaming down into the laces at Watkins Glen I had my first encounter with "brake pad or caliper knockback". Let's just say it was a code brown moment of monumental proportions I would not wish upon anyone. Thank you to the wise person on this forum who suggested months ago that this was indeed the problem. The notion of the brake pedal going right to the mat at only one spot on a track every lap really stumped me and a good number of much more experienced people.

Here's a great article describing the phenomenon in detail.

http://www.stoptech.com/technical-su.../pad-knockback

Interstingly the author had the same experience at the Glen going into the Bus Stop where as I think the Bus Stop was what precipitated my incident in the next major braking zone, the Laces.

In any case, after much discussion with my mechanic and a full season of left foot tapping the brake pedal before every major breaking zone, he has dug up and proposed a couple of solutions. I should mention that my 911 has ridiculously effective Brembo brakes (outside of this intermittent problem) installed by a previous owner. All the suspension is fresh, ie. bearings, bushings etc.

Options:

1. Caliper piston seals designed to limit the knock back, available through Brembo.

2. Anti Knockback springs, possibly also available through Brembo.

3. Brake fluid check valves.

Has anyone else had this problem and experience with these remedies? I'd be grateful for any input on this.

Thanks again.

Cheers

A.T.

[porsche930dude]

seems to me by that description that the problem lies more in the rotors or wheels or wheel bearings. Find the root of the problem and look where the flex is coming from

[KTL]

The problem doesn't lie in the rotors, wheels or wheel bearings in A.T.'s car. It's a common occurrence in even pro level racecars.

There are indeed anti-knockback springs you can install in the Brembos. Just need to make sure you get the size that fits your pistons. PFC makes springs that are for 36mm & smaller & 36mm-up in kit form

Search results for: ’knockback’

When was the last time the calipers had new pressure seals installed? New pressure seals, not just the knockback-resistant type, make a big difference in how the pistons behave. Remember the pressure seal largely supports the piston. So an aged seal is not going to "grip" the piston as well as a new seal.

[A.T.]

Thanks 930Dude Thanks Kevin.

Suspension is all good, new bearings, everything is fresh. Calipers were rebuilt with new seals last year. I've got maybe 6 DEs on them. 2 DE's on the bearings.

"It's a common occurrence in even pro level racecars." I agree. I've seen the left foot tapping the brakes down a straight in several in car videos. Here are a couple:

This guy is amazing. He does a lot of left foot braking but then I realized he was pumping the brake pedal down the straight after AueRouge as well.

https://www.youtube.com/watch?v=BrePIvGjRes

Lee Keen at the Glen

https://www.youtube.com/watch?v=BrePIvGjRes

Still curious if anyone has had personal experience with any of these fixes.

Thanks guys.

A.T.

[Eagledriver]

I got in the habit of tapping the brakes before each braking zone. That takes care of any knock back.

[Cory M]

I fought knockback for a long time (Carrera racecar with 993 RS brakes and dual masters). I finally cured it by rebuilding all of my calipers (standard Brembo seals, new pistons) and using anti-knockback springs (from Hoerr Racing).

In my case my seals all looked great visually but the calipers had sat on the shelf for a while and they had gummed up inside. It was very difficult to remove the pistons, I even damaged a couple getting them out. Fortunately the rebuild kits with new pistons weren't much more expensive than the seals alone so I just replaced them all. Hoerr Racing is selling new Brembo seals that are supposed to help with knockback now. They weren't available when I rebuilt mine but I would have used them if they were.

I don't know how much the springs help, they are very light spring rate, but they are also very cheap (~$30 total) so I don't think there is anything to lose by using them.

Very nice to have a firm pedal and no need to tap before the braking zone.

[optimus_prime]

Cory M, I'm trying to get the pistons out from my 1974 calipers, please advise what your technique was...

Thanks, Mike

[porsche930dude]

Mike iv used a greese gun to get pistons out before. Its messy though. If youve got a master cylender around and plumb that up itll probrobly work too if theyre not too stuck

[A.T.]

Thanks Corey. I'll look into Hoerr Racing.

A.T.

[Quicksilver]

I had a set of Carrera front calipers that had so much seal retraction that they were still dangerous after 2500+ miles. Good quality seals make a big difference.

[A.T.]

Hey Kevin

I've found a few more threads on this. You've commented on this topic often.

My calipers were rebuilt about a year and a half ago. I've got maybe 6 DEs on the car since then. And the rebuild was precipitated by my experience at Watkins Glen just before that. Ian, my mechanic thinks we should try the seals designed more specifically to address knockback and while we are in there we'll probably install the springs as well. How do they affect the life of the pads and rotors? Do you have any opinion on the brake line check valve idea?

Here are a couple shots of the set up. Brembo rotors up front and 930 rotors in the back.





Thanks for the feedback.

A.T.

[Cory M]

Originally Posted by optimus_prime

Cory M, I'm trying to get the pistons out from my 1974 calipers, please advise what your technique was...

Thanks, Mike

With the caliper off and the pads out. I put a block of wood in the caliper between the pistons. I remove the bleed nipple and inject compressed air into the caliper. Try to go slowly so both pistons extend. If a piston pops out it will do so with force and spray a little of the remaining fluid too, so keep your fingers clear and wear safety glasses. Try to avoid having one piston pop out and the other stay retracted, or it will be hard to get it out and you might need to reinstall the other pistons to build pressure and try again. With the pistons extended you can grab the lip (not the smooth bore if reusing) with some channel lock pliers and rotate it to work it out. Hoerr Racing (hrp) sells the proper soft tools to grab the pistons around the bore but you don't need them if you are careful. If you are replacing the pistons it doesn't matter where you grab them.

I have heard of the grease method but really don't like the idea of having anything other than brake fluid floating around in the calipers. It would be a real pain to clean up too.

Good luck.

[3literpwr]

You may want to setup a dial indicator if you can to check runout... I had kickback problems on my rally car that where related to rotor issues... This is especially scary while doing 100mph down a snow covered rally stage...

[Trackrash]

I watched a competitor at a recent vintage race shim his front rotors to achieve zero run-out. I thought this was a little over the top, since he only had .004" run-out to start. Maybe he was on to something...

[WANNA930]

You just beat me to posting the same reply.

I had a R5 Turbo 2 which had rotors that a bit excessive runout and replacements weren't easily found. An old racer buddy asked me why don't you shim them? I never heard of anyone doing it.

I spent a couple afternoons sort of blueprinting or dialing in my calipers. First was a meticulous cleaning of all mating surfaces and using a file on it's flat side truing the best I could. This alone made a difference. I then torques the lugs down to simulate operating conditions. Again a noticeable difference on the caliper (gauge)

Then I marked the rotors indicating their runout diagonal to their lug stud to give me reference. The easiest way to make the shims was to use simple aluminum cans and even high heat tape.

I will not say I wound up with zero runout but very close. The brakes had zero unwanted feel in the pedal afterward. I will tell you be prepared to spend at least a day as it is tedious work.

I think a lot of the products deal with covering what the actual problem is. Something has to cause that loss of parallelism. Eliminate what you can.

[optimus_prime]

Many thanks Cory. I tried the air pressure.. but my pistons do not move. I think they need some more persuasion than air pressure. I was wondering about using a penetrant to ease the resistance. I saw a posting recommending a penetrant made from 50/50 auto transmission fluid and acetone.. what do you think of that? The price of re-builts is daunting