W123 SLS diagnostic

[sceh]

W123 1981 diesel with 75K miles from new and an odd random knocking from the rear.
I disconnected the SLS valve control arm and then moved the lever up and down with the engine running. Nothing happened so either the hydraulic pump is giving no pressure of the valve leaks internally or something.
The spheres were changed about 2000 miles ago (but at least 10 years..)

I have not flushed the system yet but will. Can I assume that if I bleed by disconnecting the return line at the reservoir ( and putting it into another container) that the hydraulic pump works OK? I don't have the kit to test the high pressure output of the pump.

Am I right in thinking that operating the valve should raise and lower the car? I seem to recollect that it did so in the past.

thanks for any help on this very annoying problem!