I had to trace a broken door lock vacuum line in my '79 300SD. I found the break by removing both speakers in the top of the dash, and using a mirror.
Can the brake booster in a diesel Mercedes be tested in the old fashioned way? It will give a false reading if your vacuum pump is leaking or you have a leak elsewhere. If you can isolate the other leak by capping off that section, you should be able to test the booster as follows:
Booster test on other vehicles:
Test for a system vacuum leak as described below:
1. Operate the engine at idle without touching the brake pedal for at least one minute.
2. Turn off the engine, and wait one minute.
3. Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. Light application will produce less and less pedal travel, if vacuum was present. If there is no vacuum, air is leaking into the system somewhere. (most likely a faulty inline check valve, or leaky booster.)
Test for system operation as follows:
1. Pump the brake pedal(with engine off) until the supply vacuum is entirely gone.
2.Put a light, steady pressure on the pedal.
3. Start the engine, and operate it at idle. If the system is operating, the brake pedal should fall to the floor if constant pressure is maintained on the pedal.
I don't know why this test would not also apply to the Mercedes power booster.
I would give it a shot.
The booster receives it's vacuum directly from the vacuum pump. It can certainly rob a lot of vacuum with a leak. I would make sure you have good vacuum from the grey line that feeds all of the other components first.
Your year and model may be different but basically the same.
[Edited by patsy on 12-07-2000 at 12:12 PM]