Former VWDieselParts.com Forum

I've got plenty of brake pressure. Always stops good.
At the end of the day driving, the level is down a 1/4'' to 1/2''.
I can't find any leak anywhere. New wheel cylinders installed.
Any ideas? Thanks.

What kind of car??/
Early Rabbit '79ish? Check under passenger carpet.

If you don't find it in the area of the passenger area as fatmobile says, have the car sit in the driveway and push on the pedal many times and see if you can find a spot somewhere. It may a very small leak and show up now, or more later. It could be a leak that's going into the brake booster if the master cylinder is starting to go out on the main pushrod.

Sorry about that. It's a '81 pickup.
I'll check the floor mat.
I've put it up on ramps and pumped the brakes like crazy, but everything is dry underneath.
Anyway to check the brake booster or master cylinder before I pull it all apart?

Not sure about the pickups but the US built Rabbits (or at least post 1980) routed the rear brake lines outside the body. My 79 accumulated about a half quart in the rear carpet before I figured out where that leak was.

My 82 had a leak near the rear swing arm mount before it let go completely or at least I assume so since mine went down a tiny bit too before blowing the steel line in traffic one night. Glad there was a grassy median to pull onto that time.

If it's going down it's leaking somewhere and a careful inspection of all the lines is in order. I have heard of cases where fluid leaked past the master cylinder seal into the booster. I think you can loosen the nuts holding the master cylinder on enough to pull it forward and look for moisture and you can probably do that while still leaving all the lines attached.

No clue how to inspect booster first since its sealed and on the car.

But I junked an 82 jetta this year and wanted to use the booster and found my master must have pushing it in there because I tipped it over and got wet with nasty stuff

You pretty much have to pull the master cylinder to see if it's leaking into the booster.
Probably is.
Doesn't mean you need to replace the booster,.. but you'll need to clean and dry it good.
There is an o-ring on the end of the master cylinder and since the master cylinder is still working you could probably get away with just pulling it apart, cleaning it up and finding a replacment for that o-ring.
Most people would just get a new master cylinder,.. I don't think you can even get a master cylinder rebuild kit anymore.

Well I pulled out the vacuum hose from the brake booster and inserted a small rubber tube attached to a syringe and pulled brake fluid out.
So that solves where it was going. Would this be from a o-ring failure or the master cylinder?
I have a new mc I'm going to install, I was just wandering.

Bad master cylinder for sure. Hope you can dry out the booster.

Bad o-ring inside the end of the master cylinder plunger.
Nice way to check for fluid in the booster.

Double kudos on the idea to check for fluid. Pretty good thinking on that one.

Brake fluid attracts moisture, and will eat the paint off anything. So, when you change out the master cylinder, get as much of the brake fluid out of the booster as you can. Not sure if there's anything that you can put in there to slosh around to clean it out, or if this makes things a lot worse.

x2 on washing out if you can. My vaccum diapham on my stepvan usually lasts about 5 years. My rebuilder told me that the brake fluid will eat the diapham evenually. I have hooked a vacuum gauge up to it. 6 tons is hard to stop without it.

I've got a spare one on a parts truck. I think I'll just swap it out and let the other one dry out after I try to clean it.

Fatmobile wrote:Nice way to check for fluid in the booster.

Every so often I have an original thought, by accident.

So now I've put on the new master cylinder and it's leaking out both rubber plugs that the fluid reservoir goes into.
Do these need to swell up before they hold fluid?
I haven't even bled the brakes yet so there is no pressure.
There is a drip coming out both plugs about every 10 seconds or so.
What gives?

That isn't common.
I'm not sure why the rubber plug wouldn't seal around the plastic resevoir barbs.
Did you pull the rubber plugs and clean them out?
Take care with the plastic resevoir unless you have a spare. NLA.