Official: Brake Pad Knock Back Thread

[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????

[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.