Rear Caliper Pistons
 

Hi,

I'm currently in the middle of changing the rear discs and pads on my A8. Passenger side has been completed successfully.

Unfortunately, the driver side has a seized piston (the piston boot was torn) and won't wind back. The pads were also stuck in the shims and caused the disc to wear down. I'm hoping this is the cause of my low MPG issues as I've mentioned in my other thread. I have purchased a new (used) caliper and will replace it when it arrives in a couple of days.

One thing I noticed when doing this job was the offside and nearside pistons seem to be different designs. One side has 3 "cut-outs" for a piston rewind tool, but the other side doesn't. See the photos attached.

Is this normal or has someone replaced one of my calipers with the wrong part at some point? Do the pistons need turning or should they just push back? I know VCDS unwinds the electronic handbrake but they still need pushing back a little bit to give room for the new pads. As one caliper has the "cut-outs" it suggests they need turning, but there is nothing to grip on the other side.

The side I have already completed has the cut-outs and I wound it back using a rewind tool, but this may not have been necessary if they just push back.

Thanks

For future reference, once the motor has been wound back (either using VCDS or using a T40 torx bit on the back of the caliper with the motor removed), the piston can just be pushed back without turning. The replacement caliper arrived today and I used a 150mm G-clamp to push the piston back. I'll be fitting the new caliper to the car in the morning.

I've just finished fitting the replacement caliper, pads and discs to my car. A fairly easy job overall, most of the time was spent cleaning up the replacement caliper.

I've cycled the handbrake a few times and it sounds correct now (the first few times took longer than usual as expected). However, with the handbrake off and the transmission in neutral, I can't spin the brake disc (it's still jacked up off the ground).

I don't know much about the Quattro system in these cars - if the other wheel on the rear axle is on the ground, does the differential have some sort of lock that would prevent the jacked-up wheel from spinning? Or should the wheel spin freely? It feels as though the handbrake is not disengaging but it sounds like it's working correctly.

I've left it as it is until the morning. I'm hoping the new caliper is not at fault!

With permanent 4 wheel drive systems you do tend to get some diff bind so makes the brakes feel like they are stuck on somewhat

Thanks Lee, hopefully that's all it is. I'll put the wheel back on tomorrow and give it a try.

Piston is still seized unfortunately. I put the wheel back on and drove down the street and back. The caliper was smoking and it didn't feel right to drive.

The sliding pins were moving freely before I put it all back together. Seems as though the piston on the new caliper is also sticking for unknown reasons. Brake lines looked OK to me.

Just finished rebuilding my original caliper. I cleaned up the piston, and replaced the main seal and piston dust boot. It's now moving freely as it should. I'll be rebuilding the caliper on the other side soon too so they're in sync - that side also felt stiff when I replaced the discs and pads but wasn't completely seized.

If anyone's interested I can post a guide for this process, including photos and part numbers.

Yes please! good to have for future

Have just had to spend £60 getting my O/S caliper freed off.

I have been using the EHB a whole lot more than previously but it obviously didn't like being parked up for 7 days. At first operation the dash lit up like a Christmas tree and the caliper stuck open (which was lucky!). N/S worked fine.
Despite my cycling through on/off numerous times, spraying a hose on it whilst doing so and giving it a couple of 'love taps' with a rubber mallet, it wouldn't work.

Anyway local mechanic plugged the car in and cycled the pistons in and out as though the pads had been replaced, I didn't work at first so he stripped it down apparently (whatever that means), anyway all fine now.

MatthewH - Are you saying then that the problem was just the piston, and the new seals fixed the issue? was that the original symptom, that the caliper wouldn't move in or out?

I wonder if that was the issue with the replacement which you purchased too?

Out of interest, what led you to try the seals? was it self diagnosis??