126 self levelling rear suspension valve

[crowesnest]

My 1982 280SE is riding too high at the rear making the drive shafts clunk merrily.
The self levelling valve was changed for a re-kitted one but this made no difference. The original is now back in the car.
Hydraulic fluid is certainly being delivered around the system.
Opening the bleed nipple lowers the car and, by using the lever, the system will raise the car easily but it will not come back to a reasonable level.
One suggestion was that the distribution plate within the valve body is installed back to front but my mechanic says it is installed correctly.
Any clues?
PatC.