Stuck Rear Caliper Piston

P

Philip

Member
Have had a barely-worn Pagid rear inner brake pad split from its backing plate - all the friction material fell off in one piece. The caliper piston is, from the look of it, fully-extended, with just the backing plate in front of it.

I can fairly easily rotate the piston clockwise, but it won’t retract at all, even with a big caliper rewind tool (I stopped when the brace plate on the tool started to bend). Opening the bleed nipple doesn’t make any difference.

The piston is still moving to some degree when the brakes are applied, it doesn’t look corroded under the boot, and the calipers are otherwise fine.

Am I missing something obvious?
 
Hello,
Found a caliper rewind tool listed on a previous thread.
Is it similar to the one your using?
Edit.
For my benefit and understanding where we are at, could you confirm you are applying light pressure on the piston pot and at the same time twisting in an anticlockwise direction?

EDIT..

It's clock wise to wind in.

My mistake
 
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It’s a fairly standard clockwise-thread one, as so:
IMG_1291.jpeg
 
Hummmm,

thinking out aloud I guess the piston rotates in a track and there is also the rubber seal on the piston.
I guess the piston is not in its track and you may have to relocate it to start the homeward journey. Or a bit more difficult to sort out would be the rubber piston seal has become dislodged and is stopping progress.
To be safe o would take the caliper off the car and take the piston right out of the caliper to see where the problem lies.
You may need a new seal as it maybe damaged
 
Sorry for the delay I was checking the service manual for an exploded diagram

20240126_184942.jpg


Is this your caliper?
I think they only did one, the C34.

Do you have the workshop manual?
EdIt...its page 159 of the service document called Audi 2001> brake system edition 04.2002

Edit....the 3L 1.2 diesel rear calipers are different

EDIT.2
Sent a PM with link to manuals
 
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Definitely not the handbrake, the pads aren’t being pressed against the disc with it off and everything rotates freely.
 
Not the parking brake; it shouldn’t be that the piston has been too extended either. It is a floating caliper so with one pad gone and a good one in the other side it should be just 50% extended.
If the pads don’t contact the disc and everything turns that sounds correct to me. Any chance of a picture of the problem?
 
Not with the car at the moment. Agreed that one fully worn pad is the same as two half-worn pads in terms of travel - the piston is just proud of the extended boot.

The piston appears to be working (ie pressing the brake pedal presses the pads against the disc, and it isn’t binding when the pedal is released), but I can’t get the piston to retract at all to get a new inner pad in.

I can turn the piston clockwise with needle-nose pliers fairly easily, but it doesn’t appear to retract; with the caliper wind-back tool it doesn’t move at all - I gave up when the plate started to bend.
 
The dust boot should be in a locating groove near the end of the end of the piston.
The piston is located on a coarse thread with a very steep pitch. A couple of clockwise twists with snipe nosed pliers could well have fully retracted it.
It will be interesting to see pictures
 
Take a look at this, it helps to know what is inside, you can see how quickly the piston retracts with a little turn and you can see a fully extended and fully retracted cylinder:
 
@philward an interesting vid.
From the quoted symptoms it's as if the piston has come off the coarse thread since it isn't retracting but because the piston can be rotated it doesn't appear to be corroded and stuck. Perhaps a little more perseverance trying to get the piston to mesh on the thread may be required? It won't retract if the threads aren't aligned.
 

AndyBa said:
@philward an interesting vid.
From the quoted symptoms it's as if the piston has come off the coarse thread since it isn't retracting but because the piston can be rotated it doesn't appear to be corroded and stuck. Perhaps a little more perseverance trying to get the piston to mesh on the thread may be required? It won't retract if the threads aren't aligned.
Click to expand...
Yes probably applicable to your A2, however your MK2 is very similar but far more difficult to reassemble. Just saying incase you are tempted.
I think we wait for the pictures, then we will be able to see what has happened with this pad replacement.
 
Had a look today - with the caliper off, and after a lot of toing and froing with a caliper piston cube, it goes in and out very easily. Can only assume that the reset tool’s thread pitch meant that it was putting too much pressure on before the piston thread engaged.

Put half a litre of fluid through the caliper after the bubbles stopped, but the pedal’s still soft - I’ll have to try it with the pressure bleeder tomorrow.
 

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Yes, got it wound back in fine (and out, and in, a few more times just to make sure) - boot is fine, caliper cleaned up pretty well.
 
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