Brake Fluid Problem
 

1983 300D. 3 symptoms:

1) Very slow (4-6 months) brake fluid leak. Brakes work fine, but have to add brake fluid to keep rear reservoir in proper range.

2) When I turn the engine off after a long drive, I hear a bubbling noise through the firewall in front of me, i.e., where the master cylinder is. Bubbling eventually subsides about 10-20 seconds after engine is turned off.

3) Flushed brake fluid this past weekend. Rear brake flush went well, but could never get front brakes air-free. Instead, every pump of the brake pedal by my assistant resulted in same bubbling noise I hear when the car is turned off, and I could see half air half fluid going out through my clear bleeder hose and into waste receptacle. So, front brake lines are flushed, but not bled.

Despite all of these symptoms, when I drive the car, all four wheels seem to brake fine. (In other words, if the fronts weren't working, it would be easy to make the back tires skid with a hard brake, but that doesn't happen, the car just stops quickly even under hard braking conditions).

Defective master cylinder? Or could it be a bad brake booster? I don't see any fluid leaking between the booster and the master cylinder.

Any thoughts on most likely cause(s)? Thanks, fellow-MB diesel fans.

Gene

Could be leaking out the end of the M/C (the end you can't see) into the booster.

The Admiral has "Called " it!
 

Rear seal on the MC leaking into the Power Booster...
There will be all your "Lost" brake fluid.

New Booster Time.

I'll agree a new M/C or rebuild kit is in order.
Remove the brake fluid from the booster.
I doubt that has failed.

Thanks
 

Thanks for the suggestions everyone. I was thinking I would need a new master cylinder. Boosters are expensive, so I think I'll get a new master cylinder on there and see if that works by itself, and only get the booster if the new master cylinder doesn't do the trick by itself.

if you do not replace the booster, at LEAST pull it and get all the fluid out of it.
it's nasty stuff, and the booster is not designed to have fluid in it. it WILL fail!
and it's kinda hard to stop the car without it!

Does it matter what brand of brake fluid you put in a 1984 300TD? Mine is low, so I'll have to go get some. I haven't put any in before, so don't know whether the brand matters really.

^^I use Valvoline Full Synthetic Dot 3 & 4.

Any DOT 3 or 4 should work. I just use the Napa stuff, I get a good discount there. DOT 4 supercedes 3, but 5 is different stuff.