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.