The vacuum brake booster is a non-separable design, therefore, in case of failure, it is replaced as an assembly.
The performance of the amplifier can be checked as follows: in a car with the engine turned off, press the brake pedal 4–5 times. Keeping the pedal depressed, start the engine. With a working amplifier, with the start of the engine, the pedal should slightly «fail».
The non-return valve, which is located in the tip of the vacuum hose, is designed to be able to brake the car with sufficient efficiency when the engine suddenly stalls. You can check its condition by pressurizing the hose with a suitable rubber bulb. A leaky valve must be replaced.