Excessive brake pad retraction?

I'm finishing up a disc brake conversion on an old Singer, it's got all "new" hardware that's been bolted to the car for a year or so, but has been sitting for around a year or so.

It had a super beetle master cylinder on it,(3/4" bore I think) but would never bring up a good pedal. I swapped in a larger bore master cylinder from a vanagon and I think it feels better, but it could be my imagination.

I've pulled off one of the calipers to watch it in action and it appears that there is excessive pad retraction on releasing the pedal as it returns to it's original position even unbolted from the rotor.

Any ideas? I'm thinking either my MC is still too small or I have bad calipers (either from the parts supplier or from the sitting unused for so long). My first step should be to get piston diameters and do some hydraulic maths I guess.

It sounds like the seals between the pistons and caliper are twisting/binding and retracting the pistons when pressure is released. I have seen this a few times... mostly with square section seals.

Just did all the maths, (assuming 75lbs. pedal effort) should be making around 3300 lbs of clamping force at each wheel which should be plenty for this little thing. Now time to figure out what these calipers came off of and where they came from as they may still be under warranty.

stafford's kinda on the right track. the piston seal lives in a groove in the caliper housing, and is designed to deform a little bit as pressure builds in the caliper before it allows the piston to slide. when pressure is released, the deformed seal retracts back to its original shape, pulling the piston back with it.

the deformation allows running clearance off-brake, and the sliding allows compensation for pad wear.

if the seal gets stuck to the piston, it will not allow the piston to slide relative to the seal, so when pressure is applied the seal stretches farther than it's supposed to, and when pressure is released it pulls back all the way to its original shape, which causes the excessive pull-back that you're seeing.

so the answer is that the piston seals are sticking to the pistons, and it's time for a rebuild.

Are your pistons in new condition? Are they chromed? If so, they shouldn't adhere to the seal enough to pull them back.

Have you pushed the pistons out further than normal and then pressed them back only enough to get the caliper over the disc again? They may still stick there but shouldn't pull back past the 'zero' point and with use should free up on the piston unless the piston is grotty or corroded.

In some of the race cars we have real trouble getting the pistons to stay out near the rotors, but that is because of pad kick back from excessive end float on bearings/axles, not your problem as I take it we aren't talking about something you experienced in driving?

wspohn wrote: Are your pistons in new condition? Are they chromed? If so, they shouldn't adhere to the seal enough to pull them back. Have you pushed the pistons out further than normal and then pressed them back only enough to get the caliper over the disc again? They may still stick there but shouldn't pull back past the 'zero' point and with use should free up on the piston unless the piston is grotty or corroded. In some of the race cars we have real trouble getting the pistons to stay out near the rotors, but that is because of pad kick back from excessive end float on bearings/axles, not your problem as I take it we aren't talking about something you experienced in driving?

You are correct, the car is up in the air and not being driven. The calipers were new when the car was first put together by someone else and the caliper piston look chromed and shiny, but this wouldn't be the first time I've gotten a faulty brake part from an advancedpepzone box.

I did rapidly pump the pedal with the LF caliper off the rotor and it eventually crept it's way out, I squeezed it back down with some channel-locks and it still behaves the same. The other calipers seem to be working fine, so now I just need to get this one identified and hopefully swapped out for free under warranty.

I wouldn't do anything until I'd driven the car. The calipers, when fresh, will draw the pistons back further than when the seals get old and harder, and most people push WAY too hard on the pedal sitting still, and think thats where the pedal will be driving.

Also- how much is the firewall flexing?

Streetwiseguy wrote: Also- how much is the firewall flexing?

None, the brake pedal arm pivots on the frame rail under the floor. The floor pans are and firewall are plywood.

That is normal on a fresh rebuild of calipers. Drive it around, hold your foot on the pedal to get the calipers smoking hot. Should be better then. BTDT.