Hi, after 25 years of working on cars including full engine conversions, I have been defeated by a rear brake pad change.

The issue started when naively winding back the piston on my caliper, it started ok then hit a stop and wouldn't budge. I'm more inclined to understand why rather than brute force it so a quick search showed me that I need to engage maintenance mode on the EPB. I attempted to wind the EPB back with VCDS and for some reason got an error but in a moment of madness, I then clicked on the stop procedure option as I had unsuccessfully started it. I heard the motor running and went outside of the car to realise that the motor was obviously spinning until it reached a hard stop but the caliper was removed at this point so the piston just wound all the ray out and brake fluid started to seep from the piston dust seal.

I somehow managed to wind the piston back in and instead of messing around with vcds, i popped the motor off the back and wound it in manually which allowed me to wind the piston back in. I thought all was well but this is where the real issue starts.

As I had lost fluid, the brake pedal felt spongy so I proceeded to bleed the caliper. Something has gone horribly wrong and I really need help deciding what to change first. I have a pressure bleeding kit but it leaks a bit so I bled the caliper manually. Now the brake pedal is soft and when pumped a few times, will go hard and slowly sink to the bottom which I assume means I have wrecked a seal. 

Is it likely the caliper or the master cylinder?

I mean how likely is it that the MC suffered seal damage when manually bleeding the brake vs the seal around the piston becoming damaged when blindly screwing it back in whilst still on the car.

The MC looks like a pain to get to so now im thinking about changing the caliper and my next question would be. should I get a genuine used one from ebay for about £40 or should I get one of the cheap but new ones that come in at around £60, are these cheap Chinese rip offs? are they dangerous to use? Is a functioning genuine used one a better option?

Thanks for any input, the car out of action is an absolute disaster abd what should be a 30 minute job has ended up in misery and expense.

Did you pre-fill the caliper with fluid? I've found it helps a lot on VW rear calipers.

hi, thanks, I didn't but i repaired the leak in my EZbleed pressure bleed kit and used that to bleed the caliper which appeared to run bubble free.

I have since clamped the hose and the pedal still sinks. I had to use mole grips with a slit bit of hose to protect the flexi but am not sure if it was properly clamped as I didnt want to wreck the flexi. I have ordered a proper clamp but am worrid it's the master cylinder now.

Same questio though, used genuine MC is about £15, new Meyle one is £55, which is the best option?

Did you bleed all 4 lines or just the rear that popped out?

I’d be inclined to try bleeding the entire system first at all four corners and then check for leaks while the pressure is still on the system.

Then see how the pedal feels.

My grandpa used to say that even the best hunter sometimes loses its game ... oh well.

I second the above, do not despair and run a full bleed before going around to swap parts, but I think you need VCDS for that as well, if you got air inside the ABS I believe is not self-purging.

Someone here has done this job and hope chimes in.

Thanks for the input, my thoughts were that if it was air, I should be able to pump the pedal to get pressure so if it pressurises then gradually sinks to the floor, it is fluif leaking past a seal.

According to @Bap33 ’s first reply on this post, i don’t think you’re meant to wind back the caliper so potentially you’ve got a broken seal…?

Edited 7 September, 20257 Sep by travs

45 minutes ago, travs said:

According to @Bap33 ’s first reply on this post, i don’t think you’re meant to wind back the caliper so potentially you’ve got a broken seal…?

Hi,

Yes, I confirm. I didn’t wind the piston. I just pushed straight as per @ApertureS 👋 recommendations.

I'm (not) that thick, but if you got a broken seal you should have a leak, even a weep, from there.

I did %$#% a EPB caliper once by not doing the right procedure, but I was lucky not to have hydraulic damage - a 2nd hand motor was needed instead, plenty bad.

I have one of my resto projects with ruined rear seals, and they do weep, and when you act on the brakes fluid just gushes out.

Or am I missing something?

I can't see how it can be a leak in the caliper if no fluid is escaping. I have heard that when bleeding the brakes by means of operating the pedal (as opposed to pressurising the master cylinder), one should avoid pressing the brake pedal down too far as it can enter a zone where it doesn't normally go, perhaps past a bit of a wear lip, and with a non-new system this can damage the seal.

But also of course on these modern cars with ABS and auto-hold, there are more complications in the hydraulic circuit and I wouldn't like to assume it must either be the caliper or the master cylinder. I believe there is an ABS bleed function that can be activated with VCDS but can't remember the detail.

Anyway, for future reference just activate the brake maintenance function using VCDS, wait for it to stop whirring, turn everything off, then push the piston back in. Once complete turn the power back on and exit the brake maintenance mode. When you do this last step, there is lots of whirring as the system "learns" the new pad thickness. So if you haven't properly done this last step after putting everything back, the might be a problem with the parking brake effectiveness.