W211 front brake caliper overhaul ... cylinder 'sticking'?

[james2747]

I have just refurbed front brakes on 270cdi (2004). New front hoses and caliper refurb with new seals.
One of the piston rubber seals was split and hoses looked 'old' ... probably originals.
Job went ok ... just a mucky job and difficult to keep everything squeaky clean when fitting new cylinder sealing ring ... but I managed it.
Pistons and caliper cylinders were in reasonable condition ... just needed a good clean to remove some black 'deposits'.
All went back together without a problem, using plenty of clean fluid as a lubricant and even managed to get piston seals (bellows things) to locate properly in caliper and on piston at first attempt!
One side is fine and brake disc turns easily with just a hint of brake pads 'touching' the disc.
On the other side the pads are definitely being held on to the disc. The road wheel will still turn ... but definitely not as free as it should be.
Sliders are all perfect so its not a case of caliper sticking .. just feels like piston is not being released after brakes are applied.
I understood there was just enough resilience in the caliper rubber sealing ring to pull cylinder back in a tiny amount, once you take foof off brake pedal.
Question. The cylinder rubber sealing ring is not directional is it ... it looked like a square seal so I just located it in the groove in the caliper piston not bothering which way it went in first.
Any suggestions as how I 'free up' piston would be most welcome.

 

[mersum1es]

Most likely piston itself is culprit. It is very common that after chaging pads, pistons themselves are sticking. They are made some composite stuff and they not maintain their shape peroperly. When new pads put them different location, they are not moving freely any more.

 

[Blobcat]

I'm presuming you've disabled the SBC whilst working on the braking system

 

[Wighty]

If the cylinder internal rubber seal looks square in profile then it’s not directional (my 8 calipers I overhauled recently were not .
Is there any way you dislodged this internal seal when you pushed the piston in ?

 

[james2747]

If the cylinder internal rubber seal looks square in profile then it’s not directional (my 8 calipers I overhauled recently were not .
Is there any way you dislodged this internal seal when you pushed the piston in ?

Thanks for your reply.
'Eck, you did 8 calipers!!
Everything seems fine (apart from sticking) and if I had dislodged the rubber seal I doubt brakes would still be stopping me OK.
If it continues to grip ... and its not as if its a tight grip, but wheel won't spin under its own weight when you turn it by hand ... the second I stop turning it, it quickly stops. Doing same to other side, wheel will turn for at least a full revolution before it stops.
I supposeI'll have to re-do the caliper again if its same in a weeks time.

 

[james2747]

I'm presuming you've disabled the SBC whilst working on the braking system

Hi
Thanks for your reply.
Not sure the disabling (or not) of SBC would cause one caliper to be sticking while other side is fine??

 

[james2747]

Most likely piston itself is culprit. It is very common that after chaging pads, pistons themselves are sticking. They are made some composite stuff and they not maintain their shape peroperly. When new pads put them different location, they are not moving freely any more.

Hi
Thanks for your reply.
Pistons were in good condition and moved easily once I had refurbed the caliper.
I squirt a bit of air in to caliper after finishing the refurb and before re-fitting to make sure piston moves ok ... and it easily moves out. Can also push it back in with hand pressure so I'm assuming this is as free as the piston should be.

 

[Blobcat]

Hi
Thanks for your reply.
Not sure the disabling (or not) of SBC would cause one caliper to be sticking while other side is fine??

I don’t think it would, but you don’t want the system active when you’re working on it...

 

[Oldspanners]

Thanks for your reply.
'Eck, you did 8 calipers!!
Everything seems fine (apart from sticking) and if I had dislodged the rubber seal I doubt brakes would still be stopping me OK.
If it continues to grip ... and its not as if its a tight grip, but wheel won't spin under its own weight when you turn it by hand ... the second I stop turning it, it quickly stops. Doing same to other side, wheel will turn for at least a full revolution before it stops.
I supposeI'll have to re-do the caliper again if its same in a weeks time.

When the car is actually driving movement in the wheel bearings will slightly knock back the pads and allow the wheel to rotate freely. Give it a drive and try it again and try moving the wheel as if you are testing it for wheel bearing play before spinning it.

 

[joderest]

I think i would drive car for a few miles then check again, might just be the excess pad wearing off, of needs to settle after the work. If still dragging then look for a reason.
When you drive it, feel the wheel, if its very hot, more than other side, you could have a small issue.

 

[mersum1es]

Hi
Thanks for your reply.
Pistons were in good condition and moved easily once I had refurbed the caliper.
I squirt a bit of air in to caliper after finishing the refurb and before re-fitting to make sure piston moves ok ... and it easily moves out. Can also push it back in with hand pressure so I'm assuming this is as free as the piston should be.

Ok, good then.

Did you bleed them by SDS? If you have air in system, it might keep pushing piston out due to air compressability/expansion nature?