Sidney004

A little background, brakes were soft on the first push; pedal got hard after pumping it a few times. I changed the master cylinder(bench bled properly) and bled the system with my speedbleeders as I have for years. Put on a new set of rotors and some half worn PFC's with no taper at all. Inspected and measured the calipers, they are not spread.

I go to check the brakes and G......t, soft on the first push; pedal got hard after pumping it a few times. I proceed to bleed another quart of fluid thru this thing; there is no air in the lines. When the pedal gets hard after pumping it; if I wait 15 seconds it goes soft on the first push again. As a final experiment, I pop in a new set of front pads(the rears were almost new) and the brakes feel perfect.

What is going on with using these thinner pads, is air leaking past the pistons when they are extended further? Are the piston seals suspect on this set of calipers? I have never had this problem before and I typically run the pads down to the backing plates. I don't think knockback is the issue here because the car is static resting on jackstands.

CHJ In Virginia

Two things to investigate. First are the seals on the body of the Speed Bleeders OK or are they leaking? I had a problem like yours and it turned out to be the red sealing material on the body of the bleeder was worn and allowing air to re enter the system when I let up on the pedal. I got rid of them and now use a motive power bleeder - no more problems. Second - are you testing the brakes with or without the engine running? The vacuum booster can have quite an effect on the pedal feel. Check the booster and associated lines for leaks.

redtopz

Same exact thing happened to us in our lemons car which has the same brake design as the pbr calipers. We swapped new pads on old rotors (opposite of what you did but same result). After that the pedal would go to the floor but we could pump it firm. 15 seconds later is was back down to the floor. We decided to race it anyway hoping it would just go away. And it did once the pads were bedded in with the rotors. So what I think happens sometimes when you have mismatched pads and rotors is that the slight discrepancy in the mating surface gradually pushes the pistons back into the caliper a little bit causing the initial long pedal. Once they are fully bedded in it goes away.

RX-Ben

Yes. Run with it and see. I blew most of the day at Pocono chasing this - thought I had air in the master. Brakes were terrible in the paddock, rock solid on the brake check on the straight and at the first turn.

jlutherva

I have had a similar issue (C6Z) and resolved it by bolting the rotor to the hub to eliminate a very small amount of motion that, in effect, created the "Knock-back".

Jim

RX-Ben

How did that help? The rotor is held to the hub by the lugs...if the rotor is loose it seems to indicate a larger problem (interference w/brake ducts, etc).

longdaddy

pretty much every sliding caliper system I had the displeasure using on the track did this at some point or another.

trackboss

If the calipers have been used hard they have also been overheated. Cast aluminum simply gets soft and as a result the calilpers spread. It's also possible the piston bores are also out of round. If they have been powdercoated then just throw them away. That is one thing that will almost guarantee they are worthless for any hard use. One other note on factory calipers. Don't ever buy used or rebuilt ones unless it's a cast iron construction.
I still can't believe all the aftermarket shops that promote powdercoating of calipers. Just eplains how much of a lack of knowledge the operators of those shops have.

jlutherva

RX-Ben, If you were referring to my comment, I wasn't very clear. I was test the brakes w/o the wheels mounted so the rotors were loose. Securing the rotors seems to solve the problem.

I've also been guilty of not pumping the brakes long enough before concluding that there's still an air bubble somewhere. Now I pump the hell out of them before I re-bleed the brakes.

Jim

Nd4spd2

I've been considering the motive power bleeder, how do you like it?

davidfarmer

pressure bleeding is the way to go. Motive is fine, I use a nitrogen bottle with a Kent Moore cap. Sounds like there are lots of issues to investigate, but a Tech2 bleed might also be in order. I'd drive it, engaging the ABS several times, then bleed again. If you have an infra-red pyrometer, make sure all of the rotors are getting relatively hot (relative to each other).