Rear caliper issue

[thai-wronghorse]

I posted this in the mk4 section but had no replies so thought id try again in here...

I recently purchased a pair of used mk4 rear calipers to fit on my mk3,  before installing them I thought I'd give them a good clean up,  paint and general check over. Both pistons wound back easily with the proper tool and both handbrake levers move freely,  the issue I have is that one piston drives outwards with the operation of the lever and one does not. I was able to withdraw the offending piston using the tool in the opposite direction easy enough and the seals and piston are in excellent condition so I'm kinda stumped as to why it only moves outwards about 3/4mm with the lever before  returning its last position whereas the other piston will keep coming outwards?

[gazareth]

try swapping the pistons over and see what happens. they have some fancy spiral/gear thingy inside the piston that might be faulty on 1 off them or maybe just needs a good clean out.

[thai-wronghorse]

Yeah il give that a go although when I had them out they both looked to be in a very good clean condition.

[gazareth]

it would be worth a shot just to see if the fault goes to the other caliper. did you take the dust covers off and scrape out the locating grooves? they ellish for corroding up in there.

[thai-wronghorse]

If you mean the large rubber seal that goes around the piston then yes, they were both were fully removed and cleaned meticulously.

[Screech16v]

Maybe the (faulty one) will operate correctly when in normal operation against the pads ie some pressure against it.

[thai-wronghorse]

Well i had them both apart again earlier on and tried each piston in both caliper housings and it appears that one must be faulty internally as externally they both seem flawless.
inside the piston there is a worm drive piece held in with a circlip and although it will wind in and out manually with a brake wind back tool something obviously isn't right as it fails to operate when using the handbrake lever on both caliper housings.
Are these internal components available separately or is it a case replacing the complete piston (which I've seen for sale)?

[gazareth]

tbh I would just try them on the car as is. once the pressure of the brake fluid gets in behind them they may adjust up fine. or you could strip the guts of the piston and check for anything broken or sticking but I would try them on first as they are easy to pull of again if no luck.

[thai-wronghorse]

I can't see how fitting them will make any difference as this is the mechanically operated part rather than hydraulically assisted? Why would one piston work differently to another unless there was a problem somewhere along then line?

[thai-wronghorse]

Almost forgot to mention, I spoke to BiggRed brake refurbishers earlier on today and they also think there's something wrong with it but do not stock the components I require separately as they re-use all the internal gubbins.
They suggested getting another from the stealers, breakers or eGhey - Meh!