Official: Brake Pad Knock Back Thread

[Vegas69]

As some of you know I and a few others have fought brake pad knock back issues. With a fixed caliper like many of the large manufacturers use, any slack in the bearings under load cause the pads to recede the pistons into the caliper. This isn't so great the next time you hit the brake pedal since you experience more pedal travel than the previous stroke.
The first time I autocrossed my car I had to pump the brakes between the corners. I determined I had .020.021 play end play in each axle. I bought some different retainer plates and got it down to .005/.006. While that has improved it tremendously, I'm still not satisfied on the street or track. I know it's not the front because I have C6 bearing packs and they only net .001 play. At this point I called Wilwood and they don't offer a floating caliper so I would need to switch brakes if I go that route. My number two thought is to go to a tapered bearing. I called moser and they said most circle track and autocrossers run a tapered bearing but he couldn't guarantee that would fix my problem. He also said .005 was tight enough for a roller bearing.
I can't really get anybody to cough up a spec on how tight the axle end play should be. Wilwood is actually sending me some shims but how much tighter can I go and will .002-.003 really help that much. Has anyone changed from a roller to a tapered bearing and has it helped? I'm experiencing a change in brake pedal even on the street under normal drving conditions. While it's managable on the street and track. I want a consistant brake pedal. I had a brief conversation with Frank at Prodigy yesterday and it's happening with other brake manufacturers as well. He said he knows a guy that is running a 10psi residual valve in the front and back. While I'm sure that would fix my issue, I feel it will cause excess heat and pad wear on a cruise that could potentially cause brake failure.

[silver69camaro]

I'm surprised you aren't running a tapered bearing right now...but I have no idea if that would completely solve your problem though.

Don't rule out your front bearing packs. They can flex, even if they show very little runout. ZR1 bearings may be the answer if you end up not solving the problem.

[Vegas69]

If I knew what I know now....LOL I would have set the car up with a tapered bearing. I'm getting some knock back on the street as well. I just can't imagine a C6 bearing pack being a problem on the street. I'm talking about just cruising around town taking corners normally. Autocross and Road course I can see needing to go to a ZR1 hub possibly.

[rsk68]

Actually Wilwood does make a floating caliper, its there GM copy, there are two sizes a 7.06 mount center and a 5.46 mount center, with diferent models for various disk thicknes's, I had purchased just the mounts from Hiedts to mount them to the back of the Wilwood disc drum backing plate, I just havnt purchased the calipers yet. I had researched this due to the same knock back issues.

I have reduced any bearing and axle play to under .005 and knock back remains.

[Vegas69]

Thanks Rick.....are you running a tapered bearing or roller?

[Larry Callahan]

Recently Wilwood used MotiV8r to look into this problem since they are like 2 miles from my day job. They had it for a couple of days and shimmed my problems out for the most part. There is still something up but bleeding them might fix it. I can't remember what they got the play down to but I will ask.

https://www.pro-touring.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=24468

https://www.pro-touring.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=24462

[Vegas69]

Larry, have you had it on the track since? Are you running a roller or tapered bearing?

[rsk68]

I was talking to Ron Deraad at a local event, he has managed to make his front Wilwwod disks floaters and made his rear Wilwood calipers float, it didnt help his problem much, what finally fixed it for him was going to a elctrical brake booster with a larger bore, it may be the volume is just to much topush back much.

Ron chime in if I have this wrong.

[Larry Callahan]

Larry, have you had it on the track since? Are you running a roller or tapered bearing?

Roller and not on the track yet. I hope to go beat on it this weekend though.

[Vegas69]

Whatever brake knock back you have on the street will be 2-3 times worse on the track.

[RSX302]

Sorry guy's, It's been completely nuts this last month.

rsk68, you have it right. Nothing I did with knockback at the calipers helped. I built in .050" worth of rear caliper float. Nothing! I managed to confirm that it was the rear causing the problem by plugging off the rears, but everything I did would not get rid of it. I made it better by putting new bearings (tapered) and bigger Currie plates, but didn't fully cure it.

UNTIL!

Bigger bore master! The smaller the master bore, the more you notice it. I'm sure it's still there, but with my new 1 3/16" bore power assist system, you can't even tell. At the race track, it worked perfect. Brake bias felt very good all the time. I was so happy I could have cried:crying:. (Big Pvssy I am..LOL)My car has never felt so good. It took me half the day at the track to build my confidence to were it should have been in the beginning. Heal and toe was spot on into every corner. It was fun before, but now it's a blast to drive.

I drove a couple of C6 vette guy's around the track and they were blown away how well it's hooked up and stops.

I wish it was this way at the Optima event last year. I know I would have placed much better then 8th. Maybe first! LOL ...Oh well..Lifes a "B"

[RSX302]

Whatever brake knock back you have on the street will be 2-3 times worse on the track.

That's for sure...I hated that double pump crap before every corner.

[zbugger]

I know this doesn't help the thread any, but....


...I hope to go beat on it this weekend though.

lol..... That's just damn good sig material right there.

Sorry. Back to the thread now.:lurk:

[RSX302]

I know this doesn't help the thread any, but....



lol..... That's just damn good sig material right there.

Sorry. Back to the thread now.:lurk:

I heard something about Buttonwillow this weekend? Is this were Larry is going to BEAT ON IT!

I wish I could make it, but it ain't gonna happen. Maybe next one...



I like your sig! I know a night of frijoles--I'm a contributor.

[Vegas69]

The rumor is that a 10lb residual valve will fix it. I'm ready to try it!

[RSX302]

The rumor is that a 10lb residual valve will fix it. I'm ready to try it!

I tried the 2lb residual valve for the disc brake application, didn't phase it. I didn't do the 10lb because I was afraid that it would drag the rears being that it's used for drum. not sure?

[parsonsj]

I had knockback issues on mine. I finally found it was a bad front bearing pack (II Much uses C4 front bearings). My rear is 9 inch based, so the axles don't move.

jp

[Rick Dorion]

Ron, could you elaborate a bit on the booster? Thanks.

[John Wright]

I tried the 2lb residual valve for the disc brake application, didn't phase it. I didn't do the 10lb because I was afraid that it would drag the rears being that it's used for drum. not sure?
I'd run that 10# deal by Tobin before doing that....I have a feeling that it might keep your calipers clamped up enough to get some heat built up in the pads/rotors from the extra drag.

Are the sliders greased up good? Are there any excessive clearances in the bearings (like JP mentioned)or something else loose up there?

edit: nevermind, I read the OP a wee bit closer this time and you mention having the problem at the rear...I was thinking fronts....

[Payton King]

twin masters, that I think are magnifying my problem like Ron stated. Going to a power set up. I will let you know what I think.

I know knockback exists in power systems as well, since I have been hanging out on the corvette forum, but it seems most of the time it is worn bearing related. Mine are new with less than 4000 miles on them, so I am ruling those out.

[Vegas69]

I've measured my front bearing play and it's .001. Remember, I am running C6 bearing packs and Payton has C5's. This is a fixed caliper application. You will have some bearing end play no matter what. It needs measured with a dial indicator. Mine is down to .005/.006 and Matt over on Lateral G said he had .013 with tapered bearings so that wont' fix it. I don't feel I can go any tighter on bearing tolerance which means I have to figure out someway to push the pistons back. At this point, the only way I feel it can be fixed is with a power setup or a 10lb residual valve. I hate to be the test crash dummy for the residual ,but I may have to.....

By the way, Wilwood has come out with a redesigned thicker retaining plate and shims just for this problem. They were kind enough to send them to me for free. They won't help me since the shims are .010 and I already replaced mine with a full circumfrance currie plate.

[parsonsj]

My new bearing that failed had less than 1000 miles on it.

I don't see how a master cylinder has anything to do with pad knockback. But I'm willing to learn.

jp

[parsonsj]

Hey guys: Are all of you with knockback issues using C-clip axles?

[Vegas69]

John..in my application I feel the rears are the problem after measuring the end play. I've also jacked the car up after a drive and the fronts had pad drag while the backs had none. Once I reduced the slack in the bearing it improved a bunch.

I don't understand how a different master will fix it either. Hopefully Ron or Payton have some ensight. It seems to me that when the caliper pistons are knocked back .005 or .010 that the fluid is simply displaced back into the master resorvoir. My only thought is that a power system naturally exerts a certain amount of force on the master cylinder piston at all times which pushed the caliper pistons back out before the next brake stroke. Just like a 10 lb residual may do. I'd love to go inside a gm brake engineeers head for a few minutes and see just how much pressure is servicable. All we really need is someone with a power disc setup to install a pressure gauge on their system. Any takers????:idea:

[parsonsj]

I can tell you that according to my Auto Meter 2 5/8 gauges (the expensive servo motor version), I have 0 pressure readings in my system when the brake pedal isn't being applied.

I also have 0 end play in my 9 inch Ford-based axle setup (Ford really did have a better idea there). I'm thinking I would replace the C-clip setup with an eliminator kit, or go to floating calipers before I'd change masters. Nothing wrong with floating calipers (even BMW uses them on their high-end M3 and M5 machines).

I think it's true that all the people posting here about pad knock back have C-clips. But I don't know that...

jp

[gort69]

I don't understand how a different master will fix it either.

A disc/drum MC should have a RPV valve built in. Maybe that's what he was getting at.

OK - rereading his post - it makes sense. The pad knockback will still be there, but with the greater volume from the bigger bore MC it relates to less pedal travel to overcome it, the drawback being more leg effort required to slow the car.

[ponchopwr70]

Yea I have the same problems. I have bolt in axels and brand new bearings every where. I also had a problem on the front brackets, the two piece design the bolts backed out, so after I fix that problem I might throw some 2lb valves this winter while its parked. I have a lil play in the fronts, very minor so I really don't know what is causing it, but combine big dia. rotors and small master its just too easy for the pistons to retract.

[John Wright]

This is an interesting thread...I'm glad that I stumbled upon it.

[rsk68]

John,

I'm running C clips but have .005 play max.

[Vegas69]

John...we have big ford ends with bolt in axles. I emailed Tobin from Kore3 and his response was really educational. Thanks for the info on your pressures.

He said that GM designed some knock back into there systems in the 70's for less drag and fuel economy. They overcame the knock back in a manual system with a two stage piston. The large piston would take up the slack and the small piston would create the pressure needed for braking. I'm not sure which master cylinder he is referring to but I'm interrested.

The bigger piston make perfect sense now. It allows a small pedal stroke to take up the knock back as gort said. So the only option is to go to a bigger piston and power or hydraboost. I have no way of running power brakes so I'll have to go hydraboost or suffer any consequences of the residual valve. I wouldn't want any more pedal effort to stop the car with a larger piston and manual brakes.

[parsonsj]

Hmm. Well, I'm wrong about the axle end play. :)

Wilwood calipers, right?

[Vegas69]

You bet...6 piston front...4 piston out back.

[RSX302]

Ron, could you elaborate a bit on the booster? Thanks.

I'm running the electric set-up from ABS Power Brakes. I used this only because I was limited on space. The 8" master fit perfect between the firewall and my turbo. Mounted the pump in the front driverside fender. The only mild complant I have with the system is feeling the pump pulse when it turns on every 2nd or 3rd brake application. This may be how I plumbed it. Not that big of a deal, but I'll try repiping one of these days and put the accumulator between the master and the pump.

Overall it works fantastic!

http://www.abspowerbrake.com/ehpm.html

[ErikLS2]

Anyone know if this is a problem with C6 Z06 calipers? I'm using them on a 12 bolt c-clipped axle and still have to stab the axles and stuff. Would be nice to head off a problem now. Or, I could do some measuring and contribute here if it would be helpful. Might be a couple weeks though, wife just had minor surgery so I got to watch the kiddies on the weekends.

[gort69]

oops!

[parsonsj]

This just doesn't make any sense. I had pad knock back once: when my front wheel bearing failed (less than 1000 miles on it). That's it. Fix the wheel bearing, brakes are rock solid.

I don't see how you guys can have knockback with Ford big style bearings (tapered, heavy duty, right?), sealed front wheel bearings, and Wilwood calipers and rotors. I don't get it.

I'm going back to observation mode, 'cuz I got nothing to help. Good luck guys!

jp

[RSX302]

Hey guys: Are all of you with knockback issues using C-clip axles?

I'm using the Ford 9 with big tapered bearings.

I have no end play on the passenger side and .005" on the drivers side.

I didn't have knockback when I went around a lefthand corner but did in a righthand corner. Kinda following the end play's noted above.

I noticed in the tapered bearing design, bearing play within it is .005-.010". Pushing on the bearing with the bigger end plates will not get rid of this play. I haven't dug into the 9" rearend but I think you shim between the axles to reduce the endplay. Yes?No?Maybe?(not sure about this)

After floating the rear calipers with .050 play (a side), I would have thought for sure that would cure any so called knockback problems. The axle would need to move more then .025" underload. Is this possible?

Since I had other problems with stopping the car with a manual set-up. I decided to put in a power assist system. Was very nice to know that it took care of two problems with one purchase.