opm1s6

I was wondering if anyone could give me a little advice on the basic steps for the replacement of a master cylinder. I'm thinking it's pretty basic. Drain excess fluid from the reservoir, unbolt and then replace, and go in reverse. In the end I think you would need to flush your system a couple of times, or just get different colored brake fluid to make sure air doesn't stay in the system. That's really my biggest concern.

I'm pretty sure that it's the master cylinder. Anything else that could be causing the peddle to be too close to the floor?

4Driver4

Air in the system? Leak somewhere? Master cylinder failure is very rare.

opm1s6

and my indie did it on monday. he didn't have time to look at the master cylinder (he was working on timing belt) and thought that it was probably that.

i thought it was either a leak or a master cylinder problem also. he checked and said there wasn't a leak. I'm pretty sure that it isn't a leak also. It's a 86k mile A6 so I can't be that surprised.

audikillsbmw

I wouldn't go just by mileage...mine has 124k and I don't think the mast cyl has ever been replaced.

GPZ

... They can be a real bear to get primed properly if you don't.

Wheel cylinders (slave cylinders) are more likely to fail than the master cylinder... brake fluid leaks, corrosion due to not changing the fluid on the recommended interval, the fact that the cylinder pistons don't travel as far means that they don't regularily 'wipe' the inner cylinder surface, which allows crud to build up... etc.

What do you mean the pedal is too close to the floor? Is it returning all the way to the up position? Is there too much freeplay in the pedal, and if so, adjust it out on the pushrod between the pedal arm and the master cylinder... Have you taken a micrometer and measured thickness of the rotors and pads? Inspected to ANY fluid leaks? (The master and wheel cylinders, seals, fittings and lines should be bone dry, and leaks tend to attract gunk.) Have you flushed the system, not just bled it? Put at least two quarts of fluid through the system...(2 cups per wheel) to get all the crap out.

I had to replace a master cylinder on a VW once. On my Mercedes I only had to dismantle it and VERY lightly hone the bore and put it back together with the same seals, that was 4 years ago... I should probably look at that again sometime.

I have also seen excessive pedal travel on a system that didn't have the pads fully bedded into the rotors yet... couple hundred careful miles later and they got hard as a rock..

One other time I saw excessive pedal travel was when a rubber brake line was old and worn and was really expanding... you could litteraly see the hose expand and stretch when the brakes were applied. Stainless steel braided lines cleared that right up. I wouldn't expect to see this though unless the lines were either physically damaged, or excessively old. (The ones in question were 26 years old.)

4Driver4

I've never heard of anyone here replaceing one. As you say, it really isn't a maintenance item.

audikillsbmw

Could air in the ABS assembly cause an unresponsive pedal?

opm1s6

well i probably should have given you more context. So i bought the car in October (I have no clue how many bleeds have been done, although i know it was serviced regularly and was babied). Since I bought it (and since the weather has started to turn) I have been slowly upgrading parts. The brake has always been fairly soft and I do think it is traveling too far (relative to other Audi's I've driven) but it is returing to default position normally. It's definately excessive, but I don't have a metric for you right now since it's past midnight.

Two weeks ago I switched out the front rotors and pads, for cross-drilled brembo rotors (stock size, nothing BBK) with ceramic Hawk pads. The brake lines were bled, not flushed. I feel like it is just as soft, if not slightly worse (probably irrelevant since it's subjective). It's drivable, but it's smart to be paranoid about brake imperfections. I've noticed that if I don't drive the car for a day or two, when I brake for the first time, there is a slight grind for the first few brake attempts. I figure that is just normal bedding in procedure, and also from a trip I took to Vermont, which couldn't of helped the situation. I should say that after i replaced the pads and rotors I went on the highway and side roads and did a proper bedding in. I don't think that is it.

Two possibilities, I could have further exacerbated a leak problem if the caliper was hanging off the brake line. I doubt it, but with a 1999, you never know what it takes to cause a leak. I was pretty careful. Indie and I both checked my handy work and the brake lines for leaks. Nothing doing. I'll make sure to check tomorrow night again. Otherwise it has to be something in the master cylinder. I have seen on the forum, one case where the MC was loose and was traveling .5 of an inch, producing an extra 2 inches of pedal required (4.5:1 ratio?). I'll check that also tomorrow, as well as any leaks around the MC.

The MC replacement doesn't seem fun. Can you explain what you mean when you say "fill and vent the master cylinder before you installing it." How can you take it apart and clean out the gunk without completely removing it from the reservoir and the brake lines. Not only for my benefit, but for anyone who might need to replace one in the future. Danke!

BTW the Audi dominated in the Vermont snow. Absolutely dominated.

GPZ

submerge it in brake fluid and work the pushrod to try and get as much fresh fluid in as possible, and as much air out as possible... It then becomes a challenge to try and attach the resevoir and bolt the MC in place with all the brake lines and to the firewall without spilling too much fluid on your paint...

It doesn't have to be perfect, but the better it is, the easier it will be to get it bled in the end... Even though they are a 'positive displacement pump', they can be tricky to get primed and bled.

J

opm1s6

clean it out a bit and submerge it as you recommended this weekend. thanks