brakes won't hold-can't build boost

[1buick]

I was building boost from the line and I hear a bump sound and the car starts to roll. Now when ever I try to build boost, at just below 1lb the car starts to move. It never did that before. I use to build 8lbs of boost from the line and
it has a vacuum system. I replaced the master cylinder, front and rear brakes (S-10 cylinders on rear). The vacuum valves and the brake booster is all that haven't been changed. The Brakes are not hard to press. Does anyone have any suggestions. (There are no leaks to speak of). How do I check the vacuum valve on the master cylinder and the one in the line to the intake?

[1buick]

Hey guys I figured it out I replaced the booster check valve and it fixed the boost problem.