s3swiss

Got my caliper off this weekend. Looks faulty. While still plumbed in, I pressed as hard as poss on the pedal and got a squirt of fluid from the caliper. Couldn't see where it leaked from but possibly the handbrake actuating pin seal? Anyway-swapped it out for the original one which works fine.
Put the old one on the bench and tried to push the piston out with air from my compressor. The piston moves about 2cm out from fully recessed and then stops and I think air is then leaking from another seal in the caliper. I'll be stripping it down to see what is up with it. Got them via fleabay from Dewsbury Auto Salvage as re-con pair but over a year ago sadly. Will post results once stripped and checked.

Nottm_S2

the pistons come out so far and then as you say they pass the seal and so will leak (air or fluid) and you should be able to pull it by hand at that point. The (rubber) seal is set a few mm back in the caliper body

i took mine out, cleaned the piston and the caliper body this side of the seal and it went back in easily so i didnt do anything beyond that. the disc was all marked, i guess this was melted pad as it's now clean again so seems to be working for now at least

i re-greased the slider pins too

s3swiss

Thought I'd strip down my dodgy caliper today but the piston won't come out. Compressed air pushes it about 20mm out but it will go no further. Winding it out (anti-clockwise) makes no difference. Wound it pletny of times and not a mm of movement. It would seem the inner screw-thread must be turning as well so the piston is just rotating without unscrewing. Even tried levering it using the recess on the piston where the dust-seal goes but no luck. It is still firmly attached internally. Any ideas?

Dembo

Inner stud is on a bearing, so it does rotate. Unscrewing won't work.

Connect it back up; bleed the system and use the hydraulics to push it out. I had to do that with one of my stuck front calipers. If that doesn't work then I don't know what else you can do other than throw it away and get a new one.

Nottm_S2

i popped mine off using the brake pressure too

s3swiss

cheers for the replies but I'm pumping plenty of pressure through it using the compressor. The problem is that once it is fully pressurised and the piston is out as much as possible, the excess pressure is then blowing out via the handbrake actuator seal - hence why I've removed it from the car and have it on the bench. Even when on the car and under full braking (on axle stands) - it blew fluid out of the seal rather than popping out the piston fully. It seems something internal still has a firm grip on the piston (believe me, it has been under severe levering pressure to try and get it out!). Packed it away again in the 'box of bits to fix one day'

Nottm_S2

The thread is really coarse so it wont be that i reckon

Prob corrosion around the caliper outside the piston seal?

s3swiss

'Fraid not - brand new recon unit - piston is as shiny as a shiny thing (the bit I can see when I pressurise it out that is!)
Never mind - the old unit working fine and I'd refurbed it while the duff unit was on the car.
Amazing how much better the brakes are with a full set of working pistons (I also drive a '60s mini with no servo so didn't notice how bad the s2k brakes were in comparison)