You need a check valve on the manifold to booster line. This gives you a couple of brake uses if the engine quits and may be your problem.
Your booster diaphragm may also be leaking, this is a common problem. I have the same problem on my Ford fire truck and hate to have to tackle that job because it is buried under the frame. I would like to repair it but doubt that any repair kits are available. Sure would save a bunch of bucks.
