So my buddy complains of sinking brake pedal on his '83 300SD, I agree to change brake master cylinder, which was just accomplished. Bench bled the cylinder before installation, master cylinder is new. With the engine OFF, the brake pedal is firm. When the engine is running, the brake pedal is sinking, actually a bit worse than before. The brakes themselves have not been bled yet.

Is this a brake bleeding symptom, or does this suggest a leaking brake booster? I'm thinking with the engine off, than air in the brake lines themselves would manifest themself as spongy brake. When engine is running, the vacuum now adds the booster into the system. So sinking pedal with engine on points to booster, and firm with engine off suggests the brakes are fine without bleeding?

Suggestions?

'77 300D 355k mi
'95 C200 180K mi