Help! Brake Caliper NIGHTMARE

[daniel_st]

I'm pulling my freakin hair with the C5 front brake calipers - the ones with the integrated handbrake.

HANDBRAKE IS DISCONNECTED, it is NOT the issue!

The issue is no matter what I do, calipers constantly grind. Not strong enough to stop the car, but strong enough to overheat the rotors.
I wind the pistons back to bottom, install the caliper no problems, bleed the system, and when I go for a drive, the brakes start stinking.
Every time I push and release the brakes, the piston pushes, but doesn't retract!!

This has happened now twice on two different calipers with a different set of pads! Is there something special I should be doing? What is the COMPLETE procedure of winding back the piston?

I don't think I'm missing any steps, but what can explain this behavior?
I'm getting desperate.

[GiveMeABreak]

Have you replaced with NEW calipers of the correct type and when did you last change the brake fluid?

Furthermore, if and when you changed the brake fluid are you aware there are 2 procedures, 1 for bleeding the primary braking circuit and another for bleeding the secondary ESP circuit, which requires a diagnostic tool like Lexia.

[Estately]

Hi Daniel,
I know that you said that this happens with the handbrake disconnected. But you did not say if the hanbrake leaver on the caliper was free to move. If it is you will have wait for the clever guys input.
It is a while since I ran C5 MkII. However you will find a small socket, quarter drive, 5mm or 6mm (not exactly sure) will fit over the end of the leaver with the hand brake fitting disconnected. Use a short extension on the socket then work the leaver and use an oil of your choice where it enters the caliper to ensure that it moves freely.
I seem to remember a small and weak spring to "help" the leaver come back to the off position, it was not strong enough hence the oiling procedure which worked for me.
Hope that helps.
Andy.

[daniel_st]

Levers moving freely on both calipers. First time this happened I was replacing driver side caliper with an original used one. I eventually gave up, bought an aftermarket caliper from Ferodo and went to the dealership. They told me the original issue was with the used caliper. They installed the new caliper, no issues. Now I'm replacing passenger side caliper with a new aftermarket one, same brand, and new pads from Simer. And Both sides overheat.
Brake fluid is 4 months old.

EDIT: Did a re-bleeding the old-school way, and polished the pad guides, now it seems only the passenger side (the just installed new caliper) overheats. After 10 minute drive with no braking, the driver side rotor is warm-ish, the passenger side is burning hot. But the car still has inertia even uphill, so it hasn't really seized.

[daniel_st]

I have a working theory on what might be going on, and not a solution, but a workaround. (UPDATE: This is all wrong, it was the handbrake mechanism after all.)

After 4 days and 4 complete disassembles and reassembles of the calipers, and triple-checking everything else and making sure everything moves free, my conclusion is this (see attachment).

When the caliper is new or refurbished, like both calipers I had issues with, the piston slides smoothly over the piston seal ring. That's why these calipers are so easy to compress. However, that means the seal ring can't deform as much as regular before breaking adhesion with the piston, therefore the seal ring can't act as a return spring, like it normally does.
During the initial setting of the piston, when you pump the brakes, it has a relatively long path to travel to press firmly against the back of the pads. When that happens, it slides smoothly over the seal ring without deforming it. So when you release the pedal, the pressure is relieved, but there's nothing to return the piston. So it keeps pressing against the pads - very gently, since there's no pressure - but nonetheless it keeps pressing. With good pads and rotors, the friction is strong enough to cause excessive heat even with minimal pressure.

Once the caliper goes through a few hundred heat cycles in the first month or so of normal driving, the adhesion between the piston and seal ring is increased, so when the piston moves, it deforms the seal ring, and when the pressure is released, the ring pulls the piston back.

Once the caliper gets old and crusty, the seal ring gets glued to the piston, which causes it to tear if the piston moves too far. This is what happened to my original caliper which set me on the path of all this craziness

So, the workaround I found - when the caliper starts sticking - the piston has slid freely over the seal and the seal isn't acting as a spring to return it - I pull over, pull the parking brake hard, pump the brake pedal several times as hard as I can with the parking brake engaged, release the brake pedal, wait a few seconds, and release the parking brake.

And the caliper lets go.

[Gibbo2286]

I think you've got bogged down with the calliper idea but the fault is elsewhere, master cylinder most likely

[PaulC5]

What you can do is apply the brakes, then spin the front wheels to see if the pads are still tight against the discs and if so open a front bleed screw to see if the pads release. This ought to check if the fault is a master cylinder that is not letting the brake fluid return or even the brake hoses have collapsed internally.
When you fit a new front caliper I believe you should not touch the handbrake lever on the caliper until the brake pedal has been pumped to push the piston out.

[daniel_st]

Well, I've fixed them.
And we were all wrong

It wasn't the master cylinder, nor the booster, nor the seal ring.
It was the handbrake mechanism. An assembly defect.

The lever was moving freely, but they put it one notch too close during assembly, so when it reaches the stopper, it still hasn't fully retracted the piston yet.

I removed the lever, and installed it again one notch over. Been driving all day, no dragging or overheating. And the handbrake works too.

EDIT: THANK YOU to everybody for the assistance and ideas!!

[Gibbo2286]

Man made faults are always the most difficult.

[Rp0thejester]

Man made faults are always the most difficult.

I agree, kids!! Why do we have them....

[PaulC5]

Good to see this is fixed and we have all learnt a new thing to check. Were the replacement calipers made by the same company, if so they seem to be making a mistake when building them.