Does anyone know a simple procedure to test the black and white inline check valve in the big vacuum line going from the vac. pump to the brake booster? I'm guessing I'd have to disconnect the line at both ends, and plug the vacuum connections coming off of it, but then what? I don't think I could fabricate any fittings or something to attach to the ends to test with a handheld vac. pump or something.

I ask because I had the master cylinder replaced because it was leaking internally, but the brake pedal feels slightly different everyday. There was barely any fluid in the booster when I had the m/c replaced. Sometimes the pedal feels a little harder than usual, and sometimes its softer. The brakes work fairly good, but I still don't think that their stopping power is at 100%. It feels just slightly less than optimal. Could this be a vacuum-leak related problem? Everything else vacuum-related, that I can tell, in the car is working good, ACC, trans, shutting-off, etc. Oh and I recently put in new front pads/rotors, and the rear brakes aren't worn.

Oh, just wanted to mention, 1980 300D, ~140,000 miles, vacuum pump rebuilt about a year ago.

if there are two small hose barbs coming off the side of the front portion of the manifold, plug them. Then blow into the smooth inner part of the front fitting (the one that goes to the vac pump) - it should not pass your breath. Suck on it (or blow on the opposite end) and it should pass air. The purpose of the check valve is to keep some vacuum in the booster after the engine is shut off. When the engine is running it doesn't matter if the check valve is working or not as long as it allows vacuum into the booster.
The problem you are describing sounds like air in the brake lines or the booster is flaky. I haven't seen a manifold line be bad unless the small hose barbs get cracked off accidentally (easy to do!)
What brand of pads did you install? Could they be less soft than previous ones?
Are they broken in properly?
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!