This may
help
We would normally inspect the valve every 12000 miles .
In order to determine if the valve requires a service I would recommend that you carry out the "leak back test" , in order to see if it is required.
1. Simply push/displace the bottom piston up to the top of the valve either using your finger, a pen or any suitable implement, whilst doing this cover the top small vacuum nipple with your index finger.
2. Leaving your finger covering the vac nipple, remove the pen, the piston may drop approx 5mm but should then stay in position by vacuum.
3. You should now see through the bottom port that the piston is held/drawn to the top of the valve .
4. Next, remove your finger from the vac nipple. The piston should return to its seat.
5. Repeat this test several times and if there is no gradual leak back all is satisfactory and the valve can be refitted to your vehicle.
6. If however the piston moves back down, whilst you have your finger covering the vacuum nipple, the valve requires servicing. If this is the case please contact us again and we will send you a service kit.