Losing pedal every once in awhile

[Ben Banks]

Need some wisdom and experience out there to speak up. About one out of 10 times my pedal will go all the way to the floor on my 64 Chevelle equipped with Baer's 2 piston Trac-rotor, booster, master cylinder etc. I have never had problems before. I have bled my brakes at least 10 times on all four corners--which resulted in zero air release or air bubbles. I replaced the master cylinder (bled properly) and have verified that all my lines are not crimping in any way. I have vacuum at the booster and no leaks at any of the wheel cylinders (or any where else for that matter). Once again, when I lose the pedal it goes all the way to the floor but can be pumped back up. The pedal is not hard in any way and works pefectly 90% of the time.

The only things I have changed recently are my spindles. I put on CPP drop spindles with no issues EXCEPT that I think my caliper is not clearing the upper control arm in sharp turns.

Any ideas??? I'm at my whits end after I replaced the master cylinder. Thanks!

[Vegas69]

If your caliper is hitting a solid object, it is likely knocking the caliper piston back into its bore. Or, you don't have your spindles bearing tight enough and it's causing excessive brake pad knockback.

[Ben Banks]

If your caliper is hitting a solid object, it is likely knocking the caliper piston back into its bore. Or, you don't have your spindles bearing tight enough and it's causing excessive brake pad knockback.

Thanks Todd, so you are saying when my A arm hits the caliper it may be knocking the piston back into its bore. Would this cause my pedal to go to the floor?

[GregWeld]

I think it's a M/C bleeding issue.... Did you bench bleed the master BEFORE installing it?

[Ben Banks]

I think it's a M/C bleeding issue.... Did you bench bleed the master BEFORE installing it?

Yes sir, I bench bled the new one when I put it on. I was having the problem with the original Master cylinder and continue to have problems with the new one.

[GregWeld]

Okay -- Here's the next issue to check


You can get your pedal and the M/C pushrod OVER CENTER -- in other words -- Over 90*.... this will cause the piston to actually pull back - dropping the pressure off. Not a good explanation --- but basically --- put someone in the car and push HARD on the pedal -- have someone else see where the pushrod and pedal are. More than 90* and you have problems. Fix that if that's the issue and see if it still persists.

If it's not that --- then I'm stumped. Since you're having the SAME issue with two different parts -- I'm not going to blame the parts... it's got to be (well it doesn't have to be but you know what I'm saying here) somewhere in the install/system. Doubtful two different parts will give you the very same issue.


Todd (Vegas69) is an expert on pad knockback -- and that might be the issue as well --- but I can't tell you how to fix that as it's an individual car / setup / how hard you're driving etc. But he has quite a bit of information on this in his build thread - "Project Payback".

[Vegas69]

Thanks Todd, so you are saying when my A arm hits the caliper it may be knocking the piston back into its bore. Would this cause my pedal to go to the floor?

Definitely....

[Ron in SoCal]

Wouldn't that be the rotor - not the A arm - pushing the piston back in the caliper? Or Excessive play in the spindle? Sorry for the rook question, but I never knew what was causing that in my 68 till I read the official knock back thread.....

[Vegas69]

If the caliper is in fact hitting the control arm, it would easily cause the knocback the opposite way it does conventionally. Normally there is deflection in the hub which causes the rotor to push the pads back against the caliper pistons. In this situation, the caliper hits the control arm and the caliper housing gets pushed towards the rotor pushing the piston into the caliper bore. Your caliper shouldn't be hitting your control arms. That could turn into a real safety issue.

[Ben Banks]

If the caliper is in fact hitting the control arm, it would easily cause the knocback the opposite way it does conventionally. Normally there is deflection in the hub which causes the rotor to push the pads back against the caliper pistons. In this situation, the caliper hits the control arm and the caliper housing gets pushed towards the rotor pushing the piston into the caliper bore. Your caliper shouldn't be hitting your control arms. That could turn into a real safety issue.

Fortunately I am not driving it but to test the brakes. My idea was to extend the steering bump stop? Maybe extend the stop out an inch or so which would reduce my steering radius but that would prevent it from hitting my caliper and hopefully reduce the above issue as well. Sounds like the place to start! If you have any other ideas let me know-this has to be the most useful forum on the internet--thanks gents...:cheers:

[kennyd]

If your caliper is hitting a solid object, it is likely knocking the caliper piston back into its bore. Or, you don't have your spindles bearing tight enough and it's causing excessive brake pad knockback.

i had this same problem on a past project , it was the caliper making contact with the crossmember during a hard turn . pushing it off the surface, just as vegas69 said .

[Vegas69]

As long as the turning radius isn't compromised to much, sounds like the most economical fix.

[Apogee]

As long as the turning radius isn't compromised to much, sounds like the most economical fix.

What Todd said, just make sure you account for pad wear. As the outer pad wears, a floating caliper moves inboard the same amount, so you'll want to make sure that you won't make contact ever, even when the pads are ready to be replaced.

Tobin
KORE3