Leaky master cylinder

[srivett]

Well, my master cylinder seams to be leaking internally.  I pushed the pedal to the floor while bleeding some new front lines and I felt a rubbery pop through the pedal near the end of the stroke.  It did this a few times while bleeding the brakes and now if I stomp on the pedal with the engine running the pedal will slowly continue to go down.  This is a fairly new low mileage master cylinder.   :x  Does anybody know where I can get a rebuild kit for it?  Altrom doesn't have one and Bentley/VW say not to rebuild them.  Boooo

Thanks, Steve

[Justin]

i think that i have heard about a seal that is common to go bad, but i cannot remember where it is located, from what i remember it is pretty easy to change, but i have no personal expirience, some one help us both out here

later
justin

[veeman]

Yep... that "rubbery pop" and sinking pedal symptom is exactly how my old Passat felt.  The master cylinder blew out one of the seals.  I've heard that contamination can get into the system and cause this.   In my case, the MC was way old, so it was due for replacement.

I never found a rebuilt kit and luckily the replacement MC (same as for later A2's) was only around $60.  Not that bad of a price.  I believe that one was Mexican/Brazilian...there are probably German ones out there for twice that price.

Make sure you get a "bench bleed" kit (if you don't have one) to bleed out the MC before you install it.  I wish I would have.  It saves time chasing those little bubbles...

[toomanycars]

Make sure you get a "bench bleed" kit (if you don't have one) to bleed out the MC before you install it.  I wish I would have.  It saves time chasing those little bubbles...

What's a "bench bleed kit"? I'm in Australia but what sort of place may sell one? I've been having big problems with my Audi brakes. Even put in a new master cylinder but I have the pedal problem, Good, hard pedal until I start the engine...Then is goes to the floor. You an read all about it at http://www.vwwatercooled.org.au/vwforum/viewtopic.php?t=12 and  http://www.vwwatercooled.org.au/vwforum/viewtopic.php?t=41&sid=93441eb33a3171e05a2058b4a16438cc
I'm waiting for new seals for the old cylinder. I'll fit these and see what happens. I'm still intrigued about "bench bleeding".

[veeman]

Maybe this will help some...

http://www.misterfixit.com/brakbld1.htm

and

http://www.vw-resource.com/bench_bleed.html   (this one describes bleeding an MC that has already been on the car, but the procedure is the same for a new one...just skip the MC removal steps).

Basically, you just want to make sure you purge the air from the master cylinder off the car when you have access to it.  I've used old brake lines  with crimped ends, but they should sell a metric kit that does the same thing...

[toomanycars]

I've heard of bench bleeding but I've never had to do it. I usually just fill the master cyl up with fluid and bleed at the brake lines where they exit the cylinder. In the past this method has always served me well.
Thanks for the advice, I'll try to track down a bench bleeding kit and use it.

[srivett]

It appears that the rear main seal between the booster and master tends to fail but I do not know the symptoms.  I'm currently down to one front brake and the pedal falls quickly if pressing hard.  So, I'll be either buying a rebuild kit (if I can find one...) or going to the wrecker.  What do you guys think about the master cylinders attached external proportioning valves versus the plain master cylinders with a rear suspension modulated proportioning valve?

Thanks agian, Steve

[BlackTieTD]

if you are going to go with a used replacement... i just took the MC off my parts '90 jetta TD. does anyone know the size? are they interchangeable with mkI? if i don't need it, we could make a deal/trade of some kind.  

otherwise i know of a u-pull yard that will probably sell one cheap.

[veeman]

>>What do you guys think about the master cylinders attached external proportioning valves versus the plain master cylinders with a rear suspension modulated proportioning valve?

Never seen an A2 with the proportioning valve attached at the MC, but then again, I've mainly worked on A1's.  My Audi 4kq has the prop valve right under the MC and I like that design since it's not exposed to road grime and it's right there in front of you to check.  

Regardless, I'm not sure that you can swap between an MC that has that valve and one that has the prop valve under the rear beam.  The lines going to the back of the car would most likely be pretty different between the two cars.  If you were going to eliminate the rear prop valve, I'd imagine you'd have to replumb the rear of the car with new lines as they all connect to that valve.

Incidentally,  just replaced a friends prop valve in the back of his A2 GTI and it was a nightmare.  The whole thing was incredibly rusty and we ended up replacing ALL the hardlines in the car as well as the rear hoses.  Quite a lot of work considering we had to get generic hard lines and cut / bend them then re-do the flare ends.  Not impossible, but it does take some patience.

My advice would be to find a "known good" of the same type you have and go from there.  You might be able to see which models had the same MC by looking up part numbers on a world_pac site.  

Hope that helps.

[srivett]

VMan:  I got the idea of ditching the rear valving after seeing that my brothers '89 4-door diesel Golf does not have it.  It wouldn't be a problem for me since all of my rear lines need to be replaced in the near future.  The lines are so rusty that they are as big around as the fittings.   :cry:  It is quick work if you chop the lines off at the fittings and pound a 6-point socket onto the fitting.  6 lines and 2 hoses for the back should be less than 70 bucks.

BlackTie:  I'm interested if this is my problem.

Anyway, I did some more fooling around with my brakes today.  I pulled off the rear drums to look for leaks and bled all the lines again.  Nothing out of the ordinary was found.  It doesn't even look like I'm losing fluid but the pedal still descends to the floor at a slow rate.  Is it possible for a rubber line to balloon enough to cause this?  The front lines/hoses are new but the back lines/hoses are origional.  I want to be sure I've got a bad MC before plunking money down for a new one.

Thanks, Steve

[srivett]

The pedal still does the dropping thing...maybe I should get a vacuum gauge to see if the pump is working slowly.  The pedal drops when I yank the e-brake so I know the rear cylinders aren't moving much under the fluid pressure.  Atleast I'm not losing any fluid.

Steve

[toomanycars]

Similar problem, my pedal is hard and high with the engine off but sinks about 2/3 of the way down once the engine is started. No air in the system, new mc, booster is working, brakes work well but there's this long pedal travel and a "springiness" rater than sponginess in them. Checked the whole system and I can find nothing wrong. Ideas???