tester123

My Audi is never boring and always seeks my attention. Now, the rear driver side caliper is holding brake. Pulled the caliper and tried to re-seat the piston -very hard to turn. PBlaster and few mins of wrestling, it started to turn. Pumped brakes and extended the piston, sanded down whatever gunk, greased and pushed it back in. Brake seemed to work fine. I use parking brake after shutoff without even thinking. It's a habit and that messed it up again. Now stuck and gripping rotor.

At this point, I know it needs a new boot on piston and probably the seals for parking brake mechanism are shot and that's probably why all this issue. So, rebuild? or aftermarket?

Any good thread for parts required for rebuild for C5 rear brakes? Search was crashing last time I tried...

jcman

Did you look in our Vendor Directory at the top of the page, they essentially keep the forum going and are here for us.

tester123

I know about this and in fact, bought over $1000+ parts from one of the vendors. For caliper, they are pricey!. Question still stands... I saw a c4 rear caliper rebuild (https://www.audiworld.com/forums/aud...guide-2770104/). Wondering if the seal/boot kit suggested in that thread works for C5 rear caliper...

jcman

Don't know if the C4 kit will work unless it's the same part number caliper that I doubt, call some local auto parts stores for re-builds or kits, hey it's not like the rears see a whole lot of action and if you get a few years more on a 10+ car your still golden....you might even find a VW equivalent part.

tester123

I know what you are saying but the issue is that brake on that wheel is holding and rotor is getting hot, even brake smoke some times! so, I have to do something about it...

jcman

Got it, sounds like the caliper adjuster worm gear/parking brake is sticking "shot"....let it go and get a rebuilt it's the only best advise I can give with brakes.

blink

This issue happened with mine. When the pads wore past a certain point, the piston came out too far. Bought an Autozone rebuild kit, replaced the piston o-ring and the rubber boot that protects the piston. It's been fine for 6 months. If the rotors are too thin from wear, the piston will overextend again when the pads wear.

SloopJohnB@mac.com

You COULD email BigRed and ask them or check with your Audi/VW dealer.
About the only thing you really need is a correct sized O-ring.
I've rebuilt emergency brake actuators on Saab 900 cars before and it's easy with a drill press to unload and load the screw to release and install the C-clip.

tester123

Hey SloopJohnB, haven't seen you here much (heck, I've been away too for a while).. Anyway, I would love to rebuild it provided that I find the right rebuild seal kit. The boot is torn recently (my frustrated mechanic skills won ) but the main issue is with parking brake/adjuster which is probably full of rust and sticking like someone poured super glue. I freed it once but now it's due for major overhaul. Another thing is that I am not in NJ anymore where my garage has all I'd need to get it done. sucks but, I might just try to find the seal kit from $tealer.

Sounds silly and I am having a mechanic brain block; help me understand the function of the worm gear / screw in the caliper. It doesn't look like the piston turns extending out when braking. Otherwise the face of piston would show that and pads would be subjected to rotational force by the piston but pads cannot because guides would prevent that. So, why do we need to turn the piston to re-seat it again?. I know in other makes/type you'd just push them back in without turning...

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.