My 84 300SD loses it's brakes in heavy rush hour traffic. The pedal feels like it gets lower and you start to lose all vacuum assist. After running w/o traffic, all is normal. As an experiment, I tried pumping the brakes in front of my house and soon lost all power assist. I disconnected the main line to the vacuum pump and pulled a vacuum on the system by hand and it held 15" of vacuum w/o any leak down. Then, i ran the engine and measured the vacuum generated by the pump and it was 22". Any ideas out there besides avoiding rush hour traffic jams?

Thanks
Greg

Have you test the brake booster ? it could be your problem. hope this help

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MB STARS Master Guild Technician,12 years MB tech
6 years independent shop owner

How do you test the brake booster? I assume you are talking about testing for leaks?

When you ran the engine, and tested the vacuum, did you check to see what happend to the vacuum once you stepped on the brakes, and how fast it recovered? ...maybe you have a pump that is on its way to retirement, or at least could use a little TLC.
-Larry

Think Larry is correct on pump, many times the check valve comes apart & output VOLUME is reduced.