AANoying

Hey all,
Long time lurker, and very thick skulled, but I'm at wits end here and need some advice. I purchased an 02 S6 avant with an 01E swap last summer... It had issues. Brakes and wheel bearings were thrashed, among other things, anyway to the meat of the matter... When purchased it had been sitting for the better part of a year. After firing it up and driving it for a mile or so the brakes started to seize... My first thought, caliper issue. Rebuilt all 4 calipers. New pads and rotors, front and rear, no dice. Noticed some moisture in the plemumn/ battery tray region behind the motor and suspected maybe the booster had been exposed to water. Replaced the brake booster (after finding that the PN had been superseded 4 times by Audi, and that it had been changed during the manual swap with an S4 unit) replaced brake master, power bled everything. Everything is great for about a mile, at which point everything starts to get tight, and then without warning I'm stuck with a rock hard pedal and all 4 wheels are locked. Am I missing something? Is there a porportioning valve or something that could cause this? All parts brake related are new and working properly.... Payday is Friday, I'll be ordering VCDS then, as the dash looks like the Christmas tree at time square, currently EPC, check engine and ESP lights are on. ABS is working as it should as I drove it around the block in the snow the other day in hopes that cycling the abs pump may have some effect... No such luck... Any advice or insight would be greatly appreciated, and if anyone in Denver has nothing going tmrw and wants to pull codes for me, I'll buy beer...
Cheers,
Chris

SloopJohnB@mac.com

Hope not, it was a spendy fix on my 02S6 but covered under warranty.

I was thinking restricted flexible hoses to calipers but unlikely to lock all 4 wheels, therefore it's something central like the ABS unit itself or responding to bad data from the yaw sensor. Dunno if turning off the EPS (using switch on dash) eliminates the yaw sensor completely from the system.

AANoying

Thanks for the reply. Sounds like before pulling my hair out I need to pull codes... Yaw sensor would explain the ESP Situation, but wouldnt it also affect the ABS? ABS was working as it should when I brought it down a slick hill last week.. I'm going to re-examine the throw of the brake pedal and brake switch, as I suppose it's possible that when adjusting pedal height and the fact that the booster is not original to the car(nor the pedal assembly for that matter) Maybe the current setting is preventing the MC from fully returning to its rest or un-depressed position? Unlikely as the car rolls fine for a while before developing drag at the wheels... I'll post my findings, and scan results as soon as I get my cable.
Chris

jseklund

Change your brake hoses. The flexible hoses have been known to clog and create a "check valve" situation where fluid can go one way, but not the other. That would be my first guess. Brakes are mostly mechanical in my eyes....so I'm not buying into the sensor issues. Bet VCDS doesn't show any codes for this.

Once you have the VCDS - you can also check your wheel speed sensors in the monitoring blocks and make sure they work correctly. A bad wheel speed sensor will turn off the traction control.

The ABS module seems more likely to me than the yaw sensor, but those are my 3rd and 4th guesses.

Sloops advice is good in other words - I just feel like I am still leaning toward the brake lines. It may feel like they are all clamping down, but only one or two wheels are doing it, and chances are the lines are old and could use a refresh anyway.

Also, I recommend a pressure bleeder for bleeding the brakes. Fast and effective and eliminates almost all the problems I've run into in the past.

SloopJohnB@mac.com

All kinds of funny things can happen if the rod from the pedal to the brake booster is wrong. The rod should be a fixed length from the factory and matched to a car/booster but if someone used a booster from another car…

There are all kinds of kits for adding vacuum brakes to cars that didn't have it from the factory, e.g. 1968 Camaros, but they come with adjustable rods and brackets to fit the new booster to the firewall; the point being that the rod has to be adjusted for a specific car/master cylinder. I don't know if an S4 booster is workable for the car.

The oem p/n for the C5 is 8E0612107J, the B5 S4 uses 8E0612107 (no J).
I suspect there's a good reason for that.

AANoying

SloopJohnB-

So the unit I pulled from the car was 8EO-612-105J and I installed a 107J, the guy at the stealership told me that the old one was an S4 PN and I just trusted his word.... Should have known better. I Replaced the MC with 8EO-611-021 which is the same PN as the one I removed. I have to assume that everything is smushed in there as it should be as I've got good vacuum and doesnt seem to leak. Guess it's time to pull it apart again and take measurements with both new and old parts.

Jseklund:
Good point, I'll replace the flex lines as good measure, as I cannot attest to them ever being done, and it's a cheap "take that out of the equation" move. As far as bleeding goes, I'm using a Bavarian Autosport pressure bleeder, that I modified slightly to fit on an Audi resivor.
Thanks for the advice guys, I'll keep you posted

SloopJohnB@mac.com

Here's one source:
How to Perform a Brake Booster Push Rod Adjustment | DoItYourself.com

The rod on the audi brake booster is threaded…it can be adjusted on the car. Just get under there and hold the ball part, loosen the lock nut, the hold the rod part and shorted the rod by rotating the ball part. There might be a flat on the threaded rod or use a pliers as close to the threaded area as possible.

AANoying

As much as that'd make life easier, doesn't seem to be the case here, this is the old booster, with the sleeve removed... There is no adjustment I tried turning the rod multiple times in both directions and it had no effect in legnth. Seems as if the clip just holds it tight to other rod(inside the booster, that mates to the MC) there are two rods, if that makes sense. 1st pic. The only adjustment to my knowledge, is the threaded coupler that the brake pedal uses to connect it to the rod, pic 2.



No adjustment on this end, the clip is just holding it tight against the other rod, as they are two separate pieces.





This is the only adjustment I'm aware of, and merely adjusts pedal height... I backed off the brake light switch to ensure it was not preventing the pedal from coming back fully... No change.

SloopJohnB@mac.com

That is the adjustment…the threaded rod stays put, the "coupler" as you say is the ball jointed piece that threads onto the threaded rod. If the rod/coupler is too long, the pedal will push the rod into the booster too soon and also fail to retract sufficiently to allow the master cylinder to accept the fluid returning from the caliper(s) due to the deformed square O-rings in the caliper bodies returning to square. The upshot is that the pistons stay out there more and more and finally lock the rotor.

There is actually a tool and a procedure to remove the 'coupler' from the brake pedal; it looks like you've just unthreaded it from the booster rod. That's ok, but now you have to adjust the new booster correctly.

You need to ensure the pedal can come back all the way to it's stop with about ¼-½" free play, >¼" is better than ½", i.e., shorten the rod/coupler. This generally lowers the brake pedal but ensures the rod comes all the way back. Then adjust the brake light switch and cruise vent/release because otherwise the brake light won't come on as well as the cruise control won't release if you hit the brake. Also, the engine will continue to torque with the throttle…the car is set up to not respond to throttle with your foot on the brake.