81 300SD Could not wait for answers so I took apart the air pump today. Before that I tested it with the mityvac and it would not hold vacuum. There is a "peace symbol" rubber ring inside; it looked good. Nothing else looked bad and the cam continued to work the diaphragm in and out.

Put it back together. Checked again with the mityvac and still had no vacuum. Put it back in the car and it still seems to be working; in fact I can feel some vacuum when I placed my finger over the tube.

So the question - when you put the mityvac to the pump, is it supposed to hold vacuum?