1. Press the brake pedal down hard to the stop at least 5 times with the engine off, then start the engine with the brake pedal pressed. Now the pedal should move down noticeably under your foot.
2. Disconnect the hose from the brake booster if the desired effect is not achieved.
3. Start the engine. By placing your finger on the end of the hose, make sure that there is a low pressure generated by the pump when the engine is running. If there is no low pressure, check the condition of the vacuum hose, make sure that there is no damage on it. Replace the hose if necessary. Tighten all hose clamps.
Diesel Engine Cars
4. Disconnect the hose from the brake booster vacuum pump and, by placing your finger on the end of the hose, check for the presence of reduced pressure, which should be created by the pump.
If there is low pressure, measure it. If necessary, replace the brake booster (in the workshop).
(Source link is located on: RenaultBook.ru)
Attention! Change the check valve together with the booster, because if the check valve is defective, the booster membrane will be damaged by fuel vapors penetrating into the vacuum booster.