I replaced all 4 wheels with Mintex pads yesterday. The job was uneventfull! But I had what I would call an extremely difficult time pushing in the rear pistons. I have a piston tool that matches the piston indentations and attaches to a socket wrench. The pistons turned very easily but would not retract until I used my full body weight against the caliper. In the past I could retract my VW rear pistons with needle nose pliers and very little pressure. Both sides of the Audi required the same force and I have blisters on my hands for pushing/turning so hard! My original front pads had 60% lining left, the rears 10%.

First is this much force normal for a 3 year old
car used in the snow/salt? My VW GTI was 6 years old an turned in easily with needle nose pliers.

Is the 60% vs. 10% mismatched wear normal? There never seemed to be any rear wheel brake dragging.


But most of all, I would like to know exactly how that rear piston mecahnism works. If you turn it counter clockwise the piston comes out all the way immediately in a turn or two. If you turn clockwise, you can turn all day while pushing in with moderate force and it doesn't retract. Push to where you will crack hand bones and it starts going in very very slowly. At this point if you stop pushing and just turn...no more retraction. Are there threads of some sort involved here? A slip colar? This device just works very strange to me and I can't find a cross-section drawing of it.

I had the same difficulty when replacing mine, as have others, so it is not unique to your car. I, too, would love to hear an explanation of why the rear calipers work this way.

Jim Meyer
'98.5 2.8QMS

Bought the tool that (as I remember) Sears sells. Looks similar to a C-clamp; as you rotate the tool, it both turns the piston & forces it in via the screw action.

Only problem I had was the adapter that fits the cutouts on the piston; none of the Sears fittings fit that well. Had to grind one to fit the A4 piston cutouts.

Also, did you open the fluid tank when you pushed it in? On worn pads I've had more fluid than capacity, and it's needed to overflow.

On virtually all rear calipers there is a mechanical system to apply the parking brake. Now, some larger vehicles (Explorers and Grand Cherokees) have a small drum brake in addition to the caliper for parking. Audis use the integrated parking brake so the caliper does the squeezing. The parking cable pulls on a lever sticking out the backside of the caliper. When this is turned, the shaft transmits the rotary motion through some heavy duty (acme) threads and into a conical nut which seats into the piston. The conical contact force holds the nut to the piston and the whole thing gets shoved out to cause the squeeze on the rotor. The clutch has to be there with some lash so the piston can still retract after high pressure applies. When turning the piston back for new pads, a force inward has to be applied, or it will just spin on the clutch. Some calipers work a little differently, but this is the basic function. By the way a tool which pushes and turns the piston is usually not feasible, since the lead of the threads would not match. I don't know why the force is so high on these brakes.

Ray, I know what the right tool looks like from the pictures in the service manual and will make one just like it the next time I replace the rear brakes. I just thought that it was overly difficult to force in compared to VW brakes that I worked on! According to other posts here the extreme force is normal and I would believe that's why the tool is designed to apply mega force!

I understand what you say, but I sure would like a look at the mechanism in real life. I'm going to go find me a junked VW rear caliper and take it apart. I believe that they are fairly close the the A4 mechanism.

Your last comment is a little troublesome as the A4 service manual shows the tool applying both the inward force and turning the piston at the same time.

I think you have something there on the parking brake. You may have to push the piston against the parking brake stop to the point where you break your hand to engage the first thread of the piston. As soon as I take a rear VW caliper apart I will post my findings and some digital pictures.......love the digital camera for this sort of thing!

If you do a teardown on a junk caliper, it would be good to see some pics. It has been 9 years since I worked on designing brakes.