Brake Fluid Problem

[GeneE]

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