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