brake fluid squirting out reservoir top valves
 

My master cylinder was leaking on my 190D 1985. I replaced with one from a parts car and am trying to bleed the system. Hooked up my bleeder and brake fluid is squirting out the two valves on top. See pic. I pulled off the rubber caps. What are they for? Can you get the reservoir off the master cylinder . I could use the old one. But why are these valves bad and can i fix them?

Thanks

Reservoir just pulls out of the master cylinder, it takes a good hard pull. You should replace the rubber sealing grommets while its out but I have reused them if they looked real good on my personal vehicles. Relube with a wipe of brake fluid on them and the nipples and wiggle the new one back in whilst pushing.

You'll need to buy new rubber caps on the top. About a week ago I did the same procedure,(also changed the flexible brake lines) the original rubber caps were still on there and cracked. Anytime I tried to add pressure it would squirt out the valves. I put the new rubber caps on and system worked like it was supposed to. Use new grommets to save your self future headaches.

Rubber cap Part number: 000-431-90-87 only need two
Reservoir grommet part number: 000-431-09-50

Those are buttons, not valves. They are meant to test the low brake fluid bobber. If you push both of them the BRAKE light should light up.

I had that same problem a few years ago. The rubber caps crack with age; replace them with new and you'll be able to pressure-bleed the brakes.

Thanks all. Man. Sucks something so simple can stop you cold. I couldn't believe those caps are acutually what holds the brake fluid in. Not very military grade like old mercedes should be. Why couldn't they make those screw on caps ? I'm tempted to jbweld or epoxy them shut. A low brake fluid light doesn't help much when i'm driving anway. Little late at that point.

NO!!!

you don't want them sealed permanently! they are there to cover the float switches. they are SUPPOSED to be removable. no pressure is against them except when you are using the bleeder.

if you jbweld them shut, the low fluid sensors will not work.

First, fluid shouldn't squirt up like a fountain into the reservoir as you release the pedal. That is a sign that you have air bubbles in the system. Did you "bench bleed" the MC first before connecting to the car's tubes? If not, you will be bleeding for a long time trying to clear air. See youtube videos. Theoretically, it could also come from too flexible brake hoses, but they would usually burst if swelling so much.

As mentioned, those rubber caps are "test buttons" to test the "low-level warning" float switches (Owners Manual, I recall). They do not seal pressure, but they do need to seal out humidity. Any cracks and your brake fluid will absorb moisture and rust out the innards of your system. Unless, you use DOT 5 (silicone) fluid like me.

I resisted the urge to seal them. I forgot to bench bleed. Thanks Bill. The only other master cylinder i replaced was on a jeep and my friend reminded me to do that. Looks like i'm going to bleed it on the car. The Pelican should have the caps and grommets in a couple days.

"Bench bleed" doesn't mean you have to do it on the bench. Indeed, I always install the MC in the car so I can use the pedal. The important thing is that you have the outlet ports looped back to the reservoir, using clear tubing so you can see when you flush all the bubbles out. The tubes and fittings for bench-bleeding come in some rebuilt MC boxes, or auto parts usually have. If you bleed the MC well first, it is said you can connect to the car's tubes and get a hard pedal right away, as long as the tubes stay full of fluid. My trick is to slightly press the pedal as I tighten the tubes, so you get a little dribbling to insure "all liquid". For those using DOT 5, pour it down the side of a funnel so it doesn't entrain air bubbles. If you get any, wait a day for them to come out.