Servo air valve piston

My brake servo isn't working, as evidenced by the brake pedal test and having to press really hard on the brake pedal to stop the car. Having stripped down the servo the only thing I can find that may be the cause was the air valve piston which was stuck in the bore of the slave cylinder. Following the workshop manual I applied air pressure to remove the piston, but it didn't budge even up to significantly high air pressure. I eventually got the piston out with a bolt removal bit which produced enough bite to twist and pull it out.
After cleaning up the piston and bore they seemed to be in a reasonable condition, and the piston slides in the bore when inserting the reverse way round ie leaving the rubber cup outside the bore. When inserting the piston cup first into the bore it's a tight fit and I'm doubtful whether the spring tension from the air valve could push the piston down the bore. Looking at the manual it says the cup should be fitted with the lip facing the shouldered end of the piston - which I take to mean lip facing upwards to the top of the bore. However, the cup is fitted the other way round. See photo.

The odd thing is the servo has been working fine for several years until recently, without touching these components, so maybe a red herring!

Can someone confirm which way round the cup should go.


The cup should be the way it is in the photo. The other way around would see the pressure collapsing the cup and leaking into the lower chamber of the air valve.

Does the air valve have the helper spring?

Thanks Vern.
Upon investigation of the air valve assembly I found the diaphragm support missing - the bit that has the pushrod. I had changed this previously as the helper spring and main spring were broken; in fact the new one did not come with the helper spring. ( I've seen forum posts that suggest this spring is not always necessary). Presumably I forgot to re-use the old diaphragm support on the new air valve!
I will re-assemble everything and hopefully this is the cause.