[jweb]

It sounds like you could have a problem with either the check valve on the booster or the push rod length.

Check Valve - Remove the valve from the booster and blow through it. Air should only go one direction. If air goes both directions or doesn't go through at all, the valve is bad.

Push rod - Here is some good info about adjusting the push rod:
https://techtalk.mpbrakes.com/how-to...r-cylinder-gap