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Old 10-25-2000, 09:39 PM
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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?


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Old 10-25-2000, 11:20 PM
Senior Member
Join Date: Apr 1999
Location: Walnut,CA,USA
Posts: 554
Have you test the brake booster ? it could be your problem. hope this help

MB STARS Master Guild Technician,12 years MB tech
6 years independent shop owner
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Old 10-26-2000, 09:10 AM
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How do you test the brake booster? I assume you are talking about testing for leaks?
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Old 10-27-2000, 05:41 PM
Larry Delor's Avatar
What, Me Worry?
Join Date: Jun 1999
Location: Sarasota, Fl.
Posts: 3,077
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.
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Old 10-27-2000, 05:54 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Think Larry is correct on pump, many times the check valve comes apart & output VOLUME is reduced.
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