Removal
1. Loosen the clamp and disconnect the vacuum hose from the brake booster one-way valve.
2. Remove the valve from the rubber O-ring by turning it sideways and pulling it towards you.
3. Remove the rubber sealing ring from the brake booster.
Examination
1. Check the one-way valve for damage. The valve can be checked by blowing through it in both directions. Air should only pass through the valve from the brake booster side.
2. Check the rubber sealing ring and vacuum hose for wear and damage. Replace if necessary.
Installation
1. Install the sealing rubber ring into the brake booster.
2. Insert the one-way valve into the vacuum booster, being careful not to damage the rubber sealing ring.
3. Connect the vacuum hose to the valve and secure it with a clamp.
4. Start the engine and check the operation of the brake booster.
Source available here [peugeotbook.ru]