Rear Brakes ... getting 'hot'

[Jabba]

I recently replaced the rear pads and discs, used VCDS to retract the EPB - basically did it all to the letter.

The caliper pistons were difficult to get back sufficiently to allow the new pads to fit, but they did go back and it all fitted up OK. I reset the correct pad thickness in the adaptation channel, synchronized and calibrated the handbrake / EPB. No faults no issues and drives fine, can push the car, car rolls, but after a run you can smell the unmistakeable smell of hot brakes and both back wheels are hot - the front ones are not - I don't drive in a manner that really needs a lot of heavy brake activity. If you jack the car the brakes don't feel like they're binding, any ideas?

The cost of new calipers is prohibitive and it doesn't seem possible to buy new pistons - although a seal kit is available from Audi.

[snapdragon]

I had this on one side and didn't notice until it made a scraping sound as one pad was down to the metal and even though EPB warning had came on previously,
I had looked at the other side. which was not worn so reset the wear reading.
I bought the seal kit from Audi (TPS) and fitted that.
It was easy enough and gently cleaned the black deposits on piston and cylinder with some of that synthetic 'steel' wool (http://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=synthetic+steel+wool&rt=nc&LH_PrefLoc=1)and then used nearly a whole can of brake cleaner to clean out the bits followed by rinsing with brake fluid and making the caliber is clean outside to stop contamination. I needed a brake winding tool to push the piston back in (without turning the piston). The kit comes with a fluid you have to soak the seals in for a few hours before doing the job, so get them the day before you plan to do it.

[Jabba]

Well for what it's worth and because it drives me crazy when people 'fix' and don't feed back what the fix was - my fix was as above - kind of.

I replaced all the seals, this worked a treat, so boots and piston seals were the 'partial' cause of the problem. I had to clean up the pistons as they were 'corroded' and this is clearly what was preventing the piston going back easily as it was both on the piston outer surface and behind the outer seal where it sits in the caliper cylinder.

Despite the new seals one of the pistons needed too much cleaning and was too badly pitted) and on re-assembly was weeping past the piston, not much, the brakes worked OK and proved that the seals / corrosion were the problem.

What I did notice was that the pistons were different internally on each side - one had 2 flats, the other had 4, I also noticed that the threaded section that winds the piston out wasn't easily spun and each side had differing amounts of play, looks like you should be able to spin them off and back on, if you can't spin them on the threaded shaft then the servo motor is bound to be working harder than it should.

Before you start make sure that you can get the bleed nipples open, this will also help when you try to push the pistons back in, - and be gentle as the nipples will be held by galvanic corrosion (steel nipple in aluminium housing) which is an absolute pig to release. As another heads up you absolutely must NOT wind these pistons back in, they must be pushed back straight or you will mess up the handbrake mech, by winding them back you stand a chance of messing up the thread or the gears in the servo motor, if you got away with it lucky you.

To fit the piston seals you fit them into the cylinder which is fiddly because the handbrake mech gets in the way (that's how I found out mine weren't easy to spin). To fit the outer boot / dust seal you must fit it also into the groove in the cylinder BEFORE inserting the piston or it will not sit correctly in the groove designed for it. I fitted my dust seal / boot to the bottom of the piston, then got it set in the correct groove and then pushed the piston in - the piston holds the bottom of the boot nearest the cylinder in place, you will damage the boot if you do this any other way. This jobs made more fiddly by the handbrake mech and the flats on the inside of the piston which means that there are only certain rotational positions in which the piston will clear the mech, push it back too hard in the wrong position and you will have a wallet meltdown.

I couldn't use the 'official' Audi method for removing the pistons as this requires both rear wheels off the ground and both calipers removed together, Audi / VAG manual states to push the pistons out using the servo motors and provides the necessary group and setting details to accomplish this. Since I have no way to jack and support (the jack goes where the support needs to be) I settled for pushing the pistons out one side at a time with fluid pressure and hosing down the driveway afterwards.

Bottom line -

VAGCOM (VCDS) or a tool to handle the EPB correctly is mandatory - don't try rear brake work without it - or you could kill more than just the caliper (EPB ECU).
When changing your rear pads try not to push the pistons back too far, move the outer dust boot back off the outer edge of the piston and check the condition of the piston first - if it is badly corroded you will very likely get binding - or leaking. Chances are that if you're replacing pads you may need seals.
DO NOT wind the pistons back in - the piston must not be rotated - push only and only after setting the EPB in maintenance mode.
The servo motors are NOT available independently of the calipers, check them for cracks / water damage.
The caliper pistons are NOT available to 'non trade' period (in Europe at least).


My ultimate fix -- damned expensive -- since I didn't trust the hand brake mechanism on either caliper and the lack of availability of servo motors as separate items, added to the leaky piston, sods law says that if I bought caliper only and no servo motor that the motors would pack in - that's how lucky I am. I therefore bit the bullet and bought two brand new calipers complete with servo motors, not much change from £800 ....

[snapdragon]

I changed my pads a week ago and have this issue. To be honest I’m not sure if it was already present when I did the pad change as I never checked the temps - but both rear discs are hot and fronts not, even when I use the brakes minimally. One caliper is original 13 years old, the other is a new one from 3 years ago. The older one also had new seal 3 years ago as a precaution. I would have expected them to be bedded in by now.

[A8Cruiser]

Wen it comes to rear pad changing, how do you push the pistons back?
On the front ones, I use a G-Clamp. Do you do the same on the rear after putting them in maintenance/release/open for pad replacement mode?
I have previously used the 'open for pad change' mode, then wound them in some more before telling VAG-Com I had 13? mm of new pad. All was good.

Have I been doing it wrong?

[FrenchAudi]

If the pistons are pushed back after some years and several thousands of miles on often wet and salt coated UK roads, without checking their outer surfaces under the covers, there is a big chance that those parts of the piston are corroded. This will damage the seals as the pistons are pushed back, and afterwards they will tend to stick when the brakes are used. There is also a risk of pushing dirty or moist fluid back into the ABS unit.

It makes a much bigger job, but when I replace pads, if I think the pistons are dirty or corroded, I take off the callipers so I can clean or replace pistons and seals as necessary. This also enables me to completely empty the old fluid and any deposits in the cylinder, which will not come out via the bleed screw.

[snapdragon]

Good idea. As per post 4 , I replaced the seals in one of mine and then replaced the whole caliper years later. I'm not sure if the seal was replaced in the 13 year old caliper or the other one before it was replaced (as around 10 years old)
Anyhow, as the side with the original 13 year old caliper was recently running hot, I replaced the seals, cleaned up the piston and as you say the fluid was black inside despite being bled every couple of years.
All cleaned up with new inner and outer seals (which were lubed with red castor grease) all is well.

BTW, if anyone has EPB motor issues, pattern parts of the A8 version of these is now available for £39 on Aliexpress. Seal kits are about £15 on ebay.
We can now repair/refurb our rear calipers pretty cheaply.