corvuswatch

Hi! I recently replaced the front rotors and pads, and on the driver's side also had to replace the caliper. I drove it for a few days waiting for my Motive pressure bleeder to arrive. Before I could try to bleed the brakes, the pedal was soft and the brake light kept flashing but the brakes worked.

Well, I got the pressure bleeder and decided to flush the system. The brake fluid was very dirty, reddish black. After removing what dirty fluid I could from the reservoir, I put a liter of fresh fluid in the Motive and pumped it up to 20 psi (later tried it at 30). I opened the first bleeder (right front per Bentley) and barely any fluid came out, maybe 1 ounce. Even less with the left front, and none but a drip from the rears.

To help it along, I got in and pumped the brake a few times which helped only a little, but suddenly caused the brake pedal to have no resistance at all (just from the spring).

I know this 1999 AUDI A6 Avant Quattro has had the brake system seriously neglected, so I am not surprised there are more problems. Anybody have any suggestions as to what to try next?

By the way, according to Bentley, this car has the brake system that does NOT need the VAGCOM to bleed the ABS.

Thanks so much!

jcman

And it probably was from the caliper change, so do the old method of the pump and hold at each corner and yes at this point you may have to cycle the abs pump with vag com to move the fluid. Keep in mind that its not a good thing to bottom the brake pedal cause its very hard on the piston seals and can cause other problems.

SloopJohnB@mac.com

You'll need 30psi to bleed it. Chances are the bleeder you opened and nothing came out is plugged or dirty...remove the bleeder and clean it (let out the pressure in the pressure bleeder first!). Try a little pressure and see if something comes out of the caliper without the bleeder in it...if nothing comes out then the caliper is plugged. Could well be with detritus from your system in which case you can remove the caliper and blow out the cylinder from the bleeder hole or pop out the piston(s) and blow the caliper out and reinstall.

sounds like a filthy brake hydraulic system.

corvuswatch

Yes, very neglected. I would not be surprised if there are more problems yet to come. Do these symptoms seem to you to suggest that the master cylinder has failed? I am guessing so due to the peddle having no resistance and being unable to pump up. Thank you for the reply!

SloopJohnB@mac.com

master cylinders are pretty durable unless someone has mashed the pedal to the floor, literally, exceeding the normal wear area of the cylinder and put the seal(s) over crud and corrosion outside the normal wear area.

Best thing you can do is simply blow out the system with your pressure bleeder using brake fluid and then try the master cylinder. One tip..use something to hold the brake pedal halfway down when you're bleeding the system. After you're done bleeding let it up, check the master cylinder for fill level, and pump the brake pedal a few times...it should firm up.

It's also possible that the rear calipers have been effed up by the previous owner and no longer adjust correctly or the e-brake adjustment is off. They're supposed to be self-adjusting when you back up or work the e-brake level a few times.

corvuswatch

Thanks so much for the god advice. After unclogging several of the bleeder nuts, I got a good flush out of the first three brakes (FR, FL, RR), I could not bleed that last (LR) because I rounded off the nut. I did following JohnB's advice and depressed the pedal about 1/3 during the flush and bleed. Everything looked good, bubbles came out from both front brakes (just fluid from the RR).

I went to test it, and the brakes were better than before, but still very mushy, and went down to the floor at first. I could pump them up enough to get the brakes to work, but barely. After pumping them they firm up but then after 10 seconds, they go slack again. No fluid loss and no apparant leaks.

This car has 212,000 miles on it, and at this point I am guessing the master cylinder has a partial leak that allows some presure to build up, then quickly drain away.

Any opinions? If you guys concur, I'll order a new MC tomorrow. Any advice where to buy it? I prefer an East Coast vendor as it will get to me here in Northern NE faster.

Thanks again!

SloopJohnB@mac.com

You could also remove the lines from the m/c and plug them and try the master cylinder then. If it still leaks, there it is. If you get a hard pedal with the m/c plugged off, your leak is likely in the abs unit; only place left if you're not bleeding fluid onto the floor!!

some master cylinders respond to bench bleeding, i.e., put it in a vise, cap the holes, fill the reservoir and bleed the m/c by opening the holes until just fluid comes out.