Thanks for the advices here. I read the shop manual for my 300D, looking for a brake booster test, some way I can isolate the symptom. If I disconnect and successfully plug the vacuum hose coming from the vacuum pump to the booster, and tap my Mityvac into one of the tees that come off of that hose, can I use hand applied vacuum to depressurize the booster and see if it holds? Or is there a slow bleed port on the backside of the diaphram, so something else I don't know about?