I need a little help on diagnosing a brake problem on an 83, 300SD. At times, the brake pedal becomes hard and makes the car hard to stop. I really need to push on the brake pedal to stop.

I put a vacuum gauge on the vacuum circuit and I get a reading of 15 inches of hg. on the gauge. I inserted my vacuum gauge by removing a vacuum line and replacing it with the gauge. If I increase the rpm I cannot get a reading over 16 inches of hg. The manual for job 43-0023 indicates that I should have .7 to .8 bar of vacuum. I have 2/3 if that.

The car shuts down when the key is turned off, the ACC flappers appear OK and the transmission shifts OK. I get no vacuum loss in the vacuum lines at all. Any thoughts on further diagnosis?