Press the brake pedal vigorously at least five times with the engine off. Then start the engine with the pedal pressed. If the vacuum booster is in good working order, the brake pedal should noticeably drop. If this does not happen, check the tightness of the vacuum connections.
Disconnect the hose from the vacuum booster. Start the engine. Put your finger on the end of the hose and check for vacuum.
If there is no vacuum, check the hose for leaks and mechanical damage. If damaged, replace the hose. Tighten the hose clamps.
On a diesel engine, disconnect the vacuum hose from the vacuum pump and check with your finger for pressure.
If there is a vacuum, measure its value. If necessary, replace the vacuum booster.
Note: In this case, it is necessary to replace the check valve in the vacuum line, since if gasoline vapor penetrates through a defective valve, the diaphragm in the vacuum booster may be damaged.
[The information was taken from a web portal ToyotaMan]