Rear Brake Caliper Piston Rebuild
As I've mentioned before, I've been having some trouble with my rear brakes seizing. The sliding pins were fine, it turned out to be a sticking piston.
Replacement calipers are expensive even without the EPB motor, so I decided to strip mine down, clean it up and replace the seals.
The Audi part number for the seal kit is: 3D0 698 671 (around £20 inc VAT, this contains replacement seals for both sides)
Firstly, remove the EPB motor from the caliper (2 torx screws) and set it aside. The electronic connector doesn't need to be removed. Remove the caliper from the car and undo the brake line.
The brake line will continue to slowly drip fluid while the caliper is off the car, so it should be blocked somehow. Some people use grips to compress the rubber hose but I don't really recommend this.
I happened to have a spare caliper indoors so I temporarily attached it the brake line.
Ensure all brake fluid has poured out of the caliper before taking it indoors. (preferably into a container!)
The piston is now ready to be removed from the caliper. On most cars, compressed air is generally used to pop the piston out. However, as we have the electronic handbrake, we can easily remove the piston by hand. Use a torx T40 key to turn the gear on the back of the caliper anti-clockwise. This simulates activating the electronic handbrake and pushes the piston out.
Also, if you don't have a T40 key, there is one included in the Audi toolbox in the boot
As there is no disc brake to stop the piston from travelling further, it will eventually pop out.
The large dust boot on the piston can now be removed as this will be replaced. Clean up the piston and remove any dirt - I used a kitchen scouring pad. You don't want to use anything too abrasive as this could affect how well the piston seals in the cylinder.
Remove the inner o-ring inside the caliper. I used a small flathead screwdriver but be careful not to damage the inside of the cylinder. Also ensure the inner bore is clean.
The caliper is now ready for new seals. Unbox your Audi seal kit - this should contain 2 sets of seals and a bag of red fluid. The new seals must be soaked in this fluid for around 1 hour before use.
When the seals are ready, fit the new o-ring inside the cylinder and ensure it is correctly seated all the way around.
Coat the piston surface in this red fluid and fit the new dust boot in the following position:
Fit the bottom of the dust boot into the caliper. The bottom of the dust boot contains a thin ridge that sits inside the caliper bore.
The piston is now ready to be pushed back into the caliper - this is the hardest part! It must be pushed in at a perfectly straight angle otherwise it'll get stuck. I used a G-clamp to centre it and push it back in - if the angle is correct it will require very little force.
By pushing the piston all of the way in, the top of the dust boot should seat itself correctly on the piston. Push the piston in and out a few times to ensure everything is seated correctly.
The caliper is now ready to be refitted to the car. Remember to bleed the brakes.
The electronic handbrake will need to be cycled a few times to allow it to readjust before driving.