well I'm pretty sure there is no water in the system becuase i've pretty much bled my brake system about four times in the last year...and the last time I did it there was clean fluid coming out of all the calipers...I did put dot 3 fluid in it however...the thing that leads me to think it may not be that is that after driving for about ten miles without touching the brakes they'll just let loose...but as soon as I apply just the slightest bit of pressure to the pedal they're back to sticking again...and my fluid level never stays all that high in the resuvoir...if that helps at all...


AUDI: makers of the world's best automatic braking system
"All I had to do was push in on the clutch!!!!" writes Aaron from WI

 

Let see if I get this! Let's say you pull away from curb and while driving there are no brakes sticking. You apply the brakes and now you feel the brakes stick? How bad do they stick? Can you roll down a slight hill? What happens when you brakes are sticking and you let the car sit a few hours?

 

I've heard of, and had vacuum boosters fail, not apply boost to the brakes.

Is it possible for some part in the booster to actually apply permanently, the check valve, say, once you touch the brakes?

Don't wanna go get the book, just now, nor even go out to see if the '90 even HAS a vacuum booster. Will check in the AM.

If you touch the brake and it locks up, what else can it be?

Cheers,

George

 

5000/100/200 models don't use a vacuum brake booster instead they use a central hydraulic system that provides brake boost and power steering. I suspect that perhaps your troubles are somewhere with this system. After applying the brakes, perhaps the hydraulic pressure is not being released just as George has implied. I don't know if there is away to flush it. There is a procedure for flushing the steering rack, which uses the same fluid but at much lower pressure.

 

Offramp,

Good quote.

The only thing I can add is that just maybe someone in the recent past has changed the servo booster, and not adjusted the push rod to the right length. 249 m/m, 9.8 inches from the mounting flange to the center of the clevis pin on the brake pedal.

Longer, and I don't know just how much longer would be critical, and you "probably" would be partially stroking the master cylinder piston, possibly enough to cover the fluid port and not allowing fluid back into the MC as it heats and expands, and it will heat and expand on ANY vehicle, but with the port open, no problem.

I forget who said "You have to use a pressure bleeder." Though I never have, on any of my Audis, I did just read in the ABS section of my manual that "If it is necessary to bleed the brake hydraulic system after a repair operation, the system MUST (emphasis, mine) be bled using a pressure bleeding device." I never have and have not had any problems to date, but thought I would pass that along. This from my '84-'88 manual, I would suppose they're more emphatic with newer vehicles.

Sorry, I was wrong there.

Cheers,

George

BTW, where did you find that quote. Sounds like it comes from an interesting source.

Thanks

 

Same source that I always post and set up links from. Good old SJMautotechnik.com the wealth of information that this guy has posted is hard to beat. When trouble shooting my car this is the first place I look.

 

offramp...

to describe the sticking problem a little better...


here's what happens...I start driving. On short trips and trips with little braking(no city) the brakes work fine. However, as soon I start driving in the city and in turn use the brakes more the brakes start to grab little by little to the point I have to have my car in second gear to make it up hills(at apr. 30mph). the brake pedal is hard to depress and unresponsive. the car will role down a steep hill but not a slight hill...no chance. If I don't use my brakes for a while they'll let loose all at once. If I use my brakes shortly thereafter they start sticking again. If I park my car for a while and let it cool down and then drive it later on they work fine...the brake pedal goes down a little farther but they don't stick...

 

Is it possible that perhaps the brake pads/rotors might be too thick? I had a similar problem with my father’s truck. I replaced the front pads and rotors and they were sitting rather snug and everything worked. The tires rotated by hand so I didn't think much of it. We took it for a test drive and everything seemed normal. Next day my father was flagged by a motorist telling my father that there was smoke and flames shooting out through the wheels. I had to file the backing plate slightly so that there was more slop for heat expansion.

However I think that the problem is in the central hydraulic system that provides power assist. Here is what I would do. Get the brakes nice and hot, so they stick. Jack up one of the front wheels and visually check that you don’t have any binding, the caliper is supposed to free float side to side. If the pads are holding like Vice grips, very carefully crack open the bleeder valve if the fluid shoots out and frees your brakes then it is the master cylinder and power assist that is not working properly. But be very careful because you can easily get burned from hot fluid and brake parts. OR if you can get your hands on brake pressure tester gauge hook it up and check the pressure.

As George has mentioned there is a procedure for adjusting the push rod.

 

How long have you had this car, and why did the previous owner sell it?

Again with the brake servo, was it replaced?

Another point to check is to remove the cap from the brake fluid reservoir, the filter, too, if possible, watch it closely while someone else pumps the brake.

If you get a squirt of fluid, or turbulence at the surface, you will know that the port in the MC that is ALWAYS open with the pedal fully released, so that the MC has a full charge of fluid to pressurize the system.

If you are NOT getting the squirt of fluid, which indicates that the MC piston has completely uncoverd the recharge hole, it means you have a sealed system, now, and heat buildup in the fluid will apply the brakes, and make the pedal rock hard. This does indicate, either, the servo rod is too long, obstructing the hole, or the MC has failed, the piston cup has somehow come off the stem, and is not coming back far enough to oper the recharge port.

You CAN, by contorting yourself, or a friend, get a measurement of the length of the servo pushrod, without removing it. It would have to be probably at least an 1/8 inch too long to obstruct the recharge port.

I hesitate to suggest that the locknut on the servo pushrod has come loose and screwed itself OUT, the expected result would be for it to screw itself in, in that case.

Cheers,

Georg

 

I've had the car for a little over a year....as far as I know the servo has never been replaced. The guy I bought it from was selling it because he had just bought a 2000 a6 and had two other cars. He bought the car in 1991 with 21,000 miles and he pretty much told me everything that he could think of that he had done with it. he did tell me the brakes needed some work. So the brakes have stuck a little since I bought it. I assumed it was from lack of use becuase he only drove it for 6 blocks a day for two years and after I started driving it the sticking didn't really happen all that often. but they just started getting really bad in the last two months, I would assume due to the hot *** summer we've been having. So yeah, all in all, pretty sure the servo has never been replaced.

pads are definitelly not too tight. I could turn the rotors(wich are pretty worn in the first place) without a problem...

sidenote: you guys have been a big help and I really appreciate it