Originally Posted by Diesel911

Measure the vacuum as BillGrissom said. The free rantal tools at the chain autopart places should have a combination vacuum and fuel pressure gauge or just a vacuum gauge.

If it is getting vacuum some vacuum but not enough it is highly possible the mentioned check valve.
Or you can unscrew the check valve and see if the parts are still intact. If it s gunky inside spray it out with WD-40 to clean it. (that worked on mine.)

There is also an O-ring between the Mastery Cylinder and the Brake Booster that when bad causes a vacuum leak there.