To test the vacuum booster, pump the brake pedal several times to remove any vacuum from the booster. Then, press and hold down lightly on the brake pedal as you start the engine. If the vacuum booster is functioning, the brake pedal will “move downward slightly” as soon as engine starts. Mostly the vacuum brake boosters are not repaired and new are installed. Source boosters are sealed and cannot be disassembled.
This particular unit is not sealed and can be repaired. A rebuilt normally involves replacing the diaphragm valves and other plastic or rubber parts. Always refer to a service manual for exact procedures on the particular booster.