Caliper piston over-retracting

[RotoFlexible]

I am trying to cure a long brake pedal that probably results from over-retraction of two front caliper pistons (one each side). I diagnosed this with the car on stands, all wheels off. After pressing and releasing the pedal, two front pads were slightly loose while all the others were lightly touching their respective discs. When an assistant slowly pressed the pedal through its travel, I could see the loose pads come up to the discs, then back off slightly when the pedal was released.

The calipers (Plus 2 calipers on an S2, single M/C, no assist) were rebuilt about five years ago and are in good order apart from this problem. The car stops straight and pad wear is even.

I removed both calipers and rebuilt them again with new kits (the stainless steel pistons were new last time and are fine). I cleaned everything thoroughly and used plenty of lube on the piston seals (in the bore). Everything went together smoothly. I put the calipers back on the car and tested each one with compressed air. One side is much better; on the other side, the same piston as before is still over-retracting.

I have that caliper off the car again and the offending piston is out. Is there anything I can do differently or better when I reassemble it?

[robertverhey]

Hi bit of a mystery. I wonder is it worth swapping a couple of other "unhanded" components between left and right, one at a time, to try to isolate the problem. Like the pads themselves, the discs, the flexible brake lines.... Maybe the calipers themselves are fine......

[Craven]

Hi,
Sounds to me as if the piston is hanging on the seal, and not sliding cleanly through it. I know the action of the seal pulls the piston back from the disc. Have you tried new pads as this would place the seal in a different position on the piston.
Ron.

[john.p.clegg]

Agree with the above,how about some stainless polish on the pistons in a lathe and a new seal..

John

[PeterK]

Has it always had long travel on the brake pedal ?

There are two lengths of the rod that connects to the pedal, and with the shorter one the pedal will move further before dong much (about 50% travel in my Plus 2 before I changed to the longer one).
Peter

[Elan45]

Since your actual concern is a long brake pedal, have you ever considered changing up to the appropriate +2 sized master cylinder. The +2 caliper pistons are larger, so it might be logical to push more fluid to them with a larger bore master.

Roger

[alan.barker]

on my sprint i have fitted +2 calipers and kept the original MC, no problems at all.
If you fit a +2 MC pedal pressure will increase.
When you replace MC you should take the actuating rod from old MC and fit to new MC.
Sometimes on +2 ?lans you need to fit shims between pedal box and MC to eliminate excessive play.
Alan

[elanner]

Interesting thread. I have the same problem of the brakes working fine - but being surprised at the length of pedal travel when I purchased the car. I got used to it a long time ago so now it's hard to gauge how long is too long. In a non-servo car should the brakes bite immediately like a modern servoed car? Brian Buckland mentions the problem of the seals pulling the pistons back and fitting new seals to fix it.

Anyway, re the pedal actuating rod, swapping the old one over when you fit a new master cylinder is obviously the right thing to do to keep the length correct, with one caveat.

When I changed my brake master cylinder I found that the new rod was exactly the same length as the old one. However, the old one had quite a lot of wear in the clevis holes. They were more oval than round. So using the new rod eliminated a noticeable amount of loose travel. Fortunately neither the pin nor the pedal hole seemed to be worn.

There's also a bit of slop between the nose of the rod, where the pedal return spring holds it against the circlip, and the master cylinder piston. But it seems to be minimal and part of the design.

The new clutch master cylinder rod was different to the original, so I had no option other than to swap it over.

Nick

[RotoFlexible]

Thanks for all the suggestions and comments. In answer to some questions:

- With the original brakes, when the system was properly bled and the calipers were in good condition, the pedal travel was virtually non-existent and the pedal very firm. I understand that fitting +2 calipers without changing the M/C will amplify the effect of excessive clearance between the pads and the disc. The current pedal feel is a little bit of travel with constant light resistance (as the pad clearance is taken up) followed by very firm resistance and easily modulated braking effect. It's that travel I want to eliminate, to the extent possible. The pedal effort and ease of modulation are right where I want them.

- The M/C is original S2 spec, although I don't think it's the original M/C. It was rebuilt at some time and I have no reason to suspect a problem there.

- Brake lines (both hard and flexible) were replaced during the major rebuild five or six years ago. The flex lines are braided stainless over teflon.

- I wasn't clever enough to keep track of the inside and outside pistons for each caliper (although I did keep each caliper's pistons with that caliper) so I can't say for certain if the pistons are causing the problem, or the seals. Going over the sealing surface of the piston with a polish of some sort is a good idea. The seals are new. One possibility is that the groove in which the seal sits has something foreign matter in it. I tried to clean the grooves but they are hard to get at.

I'll clean up everything as best I can, reassemble, re-test, and report back. Again, thanks.

[Craven]

Hi,
As a further thought, if you have a suitable way of comparing piston diameters it may be worthwhile checking you don?t have a slightly oversize or tapered piston.
Ron.

[alan.barker]

imho you need to change the piston to the other caliper. Then you will know if the problem is piston or caliper

[billwill]

The current pedal feel is a little bit of travel with constant light resistance (as the pad clearance is taken up) followed by very firm resistance and easily modulated braking effect.

Those are the same symptoms as air in the sytem, you realise ?

[CBUEB1771]

Those are the same symptoms as air in the sytem, you realise ?

However that is not likely to account for the observed retraction of the pistons as noted in the original post. Andy, are the seal kits exactly the same for both calipers? I believe the square section of the hydraulic seal is intended to cause some amount of piston retraction upon release of hydraulic pressure. If the seals are not identical it may lead to a different amounts of retraction. How about any mounting problems that keep the piston bores from being normal to the discs. I wonder if application of the brakes is causing deflection of one disc and releasing the hydraulic force unloads the disc and pushes the piston away.

[gus]

I would bet against over retracting pistons being the issue.

I would look to the above mentioned alternatives, or a tiny air bubble or maybe runout in a disk, although you should feel that

[Chancer]

How about any mounting problems that keep the piston bores from being normal to the discs. I wonder if application of the brakes is causing deflection of one disc and releasing the hydraulic force unloads the disc and pushes the piston away.

My money is on that, when I have made my own brackets to fit Willwood calipers to Caterham uprights I had exactly that problem on one caliper where there was some distortion in the mounting.

The problem reduces when the pads wear.

You can also observe this on some mass production sliding vehicle calipers, you remove the old pads and they are worn to a wedge profile, you fit new (parallel) pads and the pedal is spongy until you fully bed them in which in extreme cases can take weeks.