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!

Moving the control arm should cause the rear suspension to move up or down, but it doesn't move very fast. Did you hold the arm in position for five or ten seconds? If you did, then I agree that the pump is probably not putting out sufficient pressure.

Putting the return line into a different container and operating the engine is a fine way to change the fluid, but it doesn't bleed any air on the other side of the valve (in the struts). To bleed that out, I like to loosen the flexible line fitting at the top of the strut, and let fluid come out until no more bubble appear. Use caution, the vehicle may drop and that fluid is at high pressure, lots of ways to get hurt if you're not careful.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

I'd also recommend that you re-start this thread in the Diesel sub-forum, far more likely to get attention from knowledgeable / experienced owners.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

The plot thickens!
I thought the struts and the whole SLS system was self-bleeding which implies that any air anywhere will bleed out.
Is it possible that there is air in my struts? This could be the root cause of the annoying knocking I get from the rear.
The car is a break (240TD) so there is access to the wheel arches above the struts I think - under a rubber cap.
As you mention, the fluid will be under pressure from the weight of the car and the air will rise so do I assume that just loosening the joint should be enough to bleed air?

I have the Workshop manual from Mercedes and curiously there is no mention of bleeding. When you change a strut you simply fill up again but looking at the diagrams, you are dead right and air can obviouly be at the top pf the strut unles there is some secret way out!