W211 Rear calipers binding & not pushing back in properly

[Muz2022]

Morning all,

I've had a brake issue, which I have now rectified .... but not solved ! Was hoping someone might be able to help.

Issue - After disc & pad replacement on rears, calipers binding on due to pistons not retracting properly after braking.

Rectification- replaced calipers.

The calipers worked fine with the old pads no issues, however the discs were shot & pads were wafer thin.
I assumed the worst & as I was short on time, I replaced the calipers.

I've since stripped the removed the old calipers with the intention of rebuilding, but can't see the issue. The caliper bore looks to be free from any corrosion/damage/debris. The pistons are ceramic and so are free from corrosion.

But the pistons will only slide nicely into the caliper about 3/4 way before jamming ‍

I've used a vernier on the pistons & can't see any out of roundness/difference in diameter etc

I seem to remember reading someone had a buildup of brake fluid residue on their pistons and had used emery cloth to sand them down ? Couldn't find that thread though

Any ideas ‍‍

 

[Muz2022]

Just actually re-read a reply to one of my posts from a few months ago where mersum said the pistons change shape after a while .......

 

[mioba]

where you get the new pads from

 

[Muz2022]

where you get the new pads from

They are Brembos

 

[AMGeed]

Did you ensure the pads are a loose fit in the caliper?

I had Brembo pads fitted in my previous W211 and they were a very snug fit. The tech had to file some paint off the sides of the pads before they fitted correctly. If they are a tight fit, they won't retract when you let off the brake.

 

[Muz2022]

Did you ensure the pads are a loose fit in the caliper?

I had Brembo pads fitted in my previous W211 and they were a very snug fit. The tech had to file some paint off the sides of the pads before they fitted correctly. If they are a tight fit, they won't retract when you let off the brake.

Hi AMgeed,

I sure did ! I lightly filed the contacting surfaces, thoroughly cleaned the caliper bracket, anti sieze compound as lubrication so no issues there.

There are no issues now that I have fitted the new calipers.

I'm trying to understand the problem with ( the now removed) calipers.

Basically the issue is that the piston is tight in the caliper when pushed any further in than 3/4.

Its a ceramic piston

There is no corrosion ... so I'm assuming the piston has somehow expanded ..... or something

 

[DREAMER NO2]

Did you release the top of the brake fluid tank ? All pads should be free to move in and out ..So many knock them into place , then expect them to move away from the disc after brake is used .

 

[Muz2022]

Did you release the top of the brake fluid tank ? All pads should be free to move in and out ..So many knock them into place , then expect them to move away from the disc after brake is used .

Hi there, yes I did !!!! and yes I agree all pads should be free to move.

I think I should have titled this thread slightly differently to read " Does anyone have experience of pistons no longer sliding into calipers smoothly"

I think I have my answers now thanks, but will leave this thread up incase anyone has the same question.

As @mersum1es has explained in other posts, the pistons can over time change shape and no longer slide into the calipers all the way.

I wouldn't have believed it if I hadn't happened to mine .... but there it is.

 

[DREAMER NO2]

So you took measurements of the caliper piston and is it distorted in some way .

 

[joderest]

1) the callipers are off the car, so no fluid to release.
It would appear from your description that the pistons have changed shape, maybe just age or something.
You could try ordering new pistons and see if they fit correctly, then re build the callipers with new rubbers etc (and give them a nice new coat of paint !) I would think new pistons and seals are cheaper than new callipers. Of course, it could be the piston chamber has changed shape, but then they would not go in as far as they do.
Failing that, just put it down to experience and bin them.