Front caliper piston off kilter on E220 CDI (W211)

[kevster]

Hi there,
I have a 2004 E220 CDI (W211).

After cleaning the front driver-side disc brake and caliper, I retracted the caliper piston using a G-clamp just enough to allow me to get everything back together again. The piston moved back. However, it didn't do such a great job, with the result that all of the piston is not sitting on the brake pad (through the gap in the caliper housing, I can see the edge of the piston projecting about 1 cm over the edge of the brake pad). I reactivated the SBC braking system afterwards and brought car for a short test drive and brakes seem to work ok. I took the wheel off to see if the caliper piston had 'readjusted' to a proper position. However, it is still projecting slightly over the edge of brake pad.

I guess my question is: if I buy a brake caliper piston retraction tool, could this straighten up the piston?

Thanks,
Kevin

 

[Tuercas viejas]

This is scary!!!
First on modern ABS brakes etc on any car you should NEVER NEVER simply compress the caliper pistons in isolation when doing brake job!

Rather you should clamp down the brake flex hose with a suitable tool made for the job, open the bleed screw, THEN with a tool like a caliper piston compressor, squarely depress the pistons into the caliper to bottom them out!
This will ensure the piston(s) displace all the contaminated fluid out of the caliper body and not back flowed up stream into the ABS brick where it can cause serious problems with the valves etc that dwell inside there.

Then re-fit the caliper using new brake hardware and new caliper retaining bolts and loctite if they are not the "stickly" variety then torque to spec.

Having got it all back together conduct a brake bleed flush out, expelling the old fluid in the usual way
Once you have fresh fluid throughout the whole brakes system. Apply a full, pedal force and then check the contact & operation of the brakes etc.
Then once satisfied you have done a good job do a road test to "bed; the pads to the disc rotors .

Hope this is helpful in resolving your problem.
Tuercas Viejas

 

[Auto-mobile]

I can't see there being a problem if the caliper has come back out again, you would loose braking to one side if it didn't touch the pad at all, (if I am reading that right)

As for the essay above, I have been working on cars for nearly 20 years and to date have never had a problem using large pliers to squeeze the piston back, to open a bleed nipple to decompress a piston in a caliper is just going to risk introducing air into the system.

 

[Bigdw]

In the WIS manual, it shows a tool to retract the calliper, dosent say anything about clamping the hose and undoing the bleed nipple

 

[grober]

Mercedes-Benz W211 Brake Pad Replacement (2003-2009) E320, E500, E55 | Pelican Parts DIY Maintenance Article

 

[The Mirror Man]

OP: can you elaborate a little.
"with the result that all of the piston is not sitting on the brake pad"? Do you mean the piston is not touching the back of the pad or only part of the piston is touching the pad?
"the edge of the piston projecting about 1 cm over the edge of the brake pad"???? What do you mean by "the edge of the brake pad"?

 

[kevster]

I've attached a picture showing how much the piston is offset from the edge of the brake pad, about 1cm. There should be no such offset - the entire piston should be in contact with the back of the brake pad. So, I was wondering will the piston 'right' itself after some use or would a piston retraction tool get it back in line?

 

[Trickythemerc]

Looks like the pads are either incorrectly fitted, or the wrong pads have been supplied.

 

[kevster]

Thanks guys. I'll take a look later and double check I've the pads back in correctly.

 

[MancMike]

Looks like the pad is in upside down to me...

Not rear pads fitted to the front are they?

 

[kevster]

They are the same pads that have been on the car since I bought it a few years ago (that's not to say they are the correct ones!) I'll check to see that I've the pads in properly. I remembered to etch 'outside' onto the outer pad when I removed it.

 

[exvolvoman]

Looks as though the pads are too small, or not correctly fitted, are both pads the same size and have you checked the other side?
The caliper piston should not overlap the edge of the pad at all.
Also, when pushing the piston back into the caliper, remove the master cylinder cap to allow fluid to escape if it needs to...

 

[pmcgsmurf]

Also, when pushing the piston back into the caliper, remove the master cylinder cap to allow fluid to escape if it needs to...

Not applicable to the SBC system at all.

I'd read up a lot on this if the op is not familiar with it.

Had stuck pistons on the front of my W211 before causing uneven brake application, new caliper required.

 

[John N]

Looks like the pads are either incorrectly fitted, or the wrong pads have been supplied.

My sentiments exactly!