[Bruce Edwards]

Ron,

The check valve is the part in the booster that the hose hooks to. Small plastic part on most. Take it out and see if you can blow air threw it both on both sides. If you can it is bad. Some get stuck open and the brakes will feel ok at first and start to go away and get harder as you slow down. Another way to check is to start the car then shut it off and after a few minutes step on the brakes. If it is still soft the valve is good.
Let me know what you find.