Welcome to the forum.It sounds to me like your booster is leaking. When you are testing is the engine running or are you pulling vac with your MityVac, or runnnig the engine to reading it? The way I do it is to plug the MityVac into on of the T's. The plug the other one. Then start the car. If all is well you'll get over 20" of vac. If not then pull the main vac line off the booster and plug that end. A thumb usually works well, if you can get it to seal that is. If the vac doesn't jump up then the pump is bad, which probably isn't your case. If it goes up then the booster is leaking.

__________________