Start the engine and shut it off after one or two minutes. Slowly press the brake pedal several times. If the pedal goes all the way down after the first push and gradually rises after the second and third push, then the booster is leak-tight.
Depress the brake pedal with the engine running and stop the engine with the pedal depressed. If, after holding the pedal down for 30 seconds, there is no change in the pedal stroke, then the amplifier is sealed.