Originally Posted by pr0xZen

To the best of my knowledge, the piston itself doesn't seat on the threaded rod - it sits "on top" of the self-adjusting contraption (don't know the appropriate english term) - which IS seated on the threads. The piston is floating freely. When "adjusting", the hydraulic pressure pushes the adjuster outwards (thus turning), and this in turn pushes against the outer piston. It will stop turning when it encounters enough resistance, and thus friction, between the adjuster and the piston. So the lack of resistance -> friction between the adjuster and piston, allows the adjuster to rotate when pushed outwards. There is a little leeway, so that the piston can move and return for normal operation. When the pads & rotors wear, there is again low enough resistance for the adjuster to move a little more.

There is no worm gear in there (in these ones anyway), the lever mechanism that the cable rotates, is shaped like a cam. When you engage the parking brake, what the mechanism does is push the entire threaded rod assembly outwards (and there is a powerful return spring inside, pulling it back when disengaging). With age, heat etc - lubrication will suffer. If the dust boot here goes, rust and dirt will start to seize up this mechanism, and and some point the friction will overpower the return spring. Blown up parts diagram tells alot of the story (here).

Now in an ideal scenario there would be zero friction between the piston and adjuster, and thus you'd be able to push the piston and the adjuster would rotate and "screw back in". But in the real world, metal-on-metal is never frictionless. When you're applying force, you're inducing a temporal friction bond between the piston and the adjuster, and thus turning the piston turns the adjuster. The square-cut seal normally applies an initial force. Also, if the setup had somehow been completely frictionless, the adjuster wouldn't work as it does - it would completely lock up the brakes.

Some patents involve notches or locking shims on the adjuster and/or inside the piston, enabling the piston to rotate freely in one direction - this does away with the friction bond element when compressing the piston/adjuster assembly.

I found a youtube clip that shows some of this, might explain this better than my words: -here-

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Depending on condition, rebuild or replace is a matter of savings vs work/headaches. If the piston dust boot has been knackered for some time, rust is likely to have started eating at the piston. If this is the case, even if sanding the piston and changing the squarecut seal (and dust boot), its not going to last very long. It'll eat those squarecuts fairly rapidly. At that point you're looking at a new piston to add to the bill.

If the dust boot for the parking brake adjuster has gone, you're likely looking at a real party, and the parts cost is closing in on "aftermarket caliper" prices. This guide -here- is for the C4, but the rough principles are the same.

If your Audi is a keeper, the piston rusted and/or the parking brake mechanism is severly compromised - I'd say get a new caliper. Preferrably from someone that do core exchange, but whatever floats your goat / is cheaper. If aftermaket parts are ok with you, then the little you'll save on DIY'ing a complete refurbishment of the rears is not worth the labour and disassembly/assembly headaches. Well, atleast if you're not very well versed and drilled in the procedure.

Originally Posted by tester123

Part stores are quoting around $95+ around here and even if the seal kit costs $30, the rest $60 is easily eaten up in the form of hour(s) of labor I'd have to put in.

Originally Posted by pr0xZen

~$100 for a new caliper is really not a bad price. Just check your gliders etc, those usually don't come with. If the pins are good, new boots for those are barely a few $. Use silicone grease, it stands up to the heat better. If it turns out you're brake piston is rusted, an aftermarket seal kit with a new piston is going to set you back atleast $30. If the parking brake assembly needs work, those parts add up too. So yeah - those (best case scenario) $60 you save, is really not worth the hassle.

Originally Posted by SloopJohnB@mac.com

It's really irritating if the rebuilder DOESN'T clean/rebuild the emergency brake components. IMHO a rebuild should involve complete disassembly, cleaning, and replacement of all elastomeric components. Sometimes all the 'rebulder' does is blow out the piston, clean the caliper, replace the piston seal and dust boot and call it a day. I've had several rebuilt OG900 Saab front rebuilt calipers that I've had to rebuild the correct way. Returning the caliper for another one didn't help and it just aggravated me.

I'm not saying these cars are so old that they're classics and you have to do it yourself, but maybe...

Originally Posted by tester123

I agree with you both. Personally, I'd prefer rebuilding. Today everyone is closed so, I'll call around tomorrow and find out more.

Originally Posted by nickcarguy

For what it's worth, my rear right caliper parking brake actuator was handing up and not completely releasing. It was not the cable. It was the internal parking brake pusrod mechanism. Normally I would go with new OEM, but after speaking with the folks at German Auto Parts I opted to replace with a A1 Cardone Remanufactured caliper.
It looked brand new , shot blasted and refinished, came with all new hardware and instructions to return the Core for refund of the deposit.
Saved about $100 dollars off Noew OEM, comes with a warranty, and works perfectly.
ReMan may not be for everyone....but in this case on a 15 year old vehicle it was the right choice.
Good luck.