My understanding from comments on this and other MB lists is...

With engine off, load cargo, rear end drops due to heavy load. Turn engine on, pump activates self leveling and rear end rises to unloaded height. You can manually operate system with controller with engine on and rear will go up & down. However, if there is a problem in the system, it will not rise with cargo loaded. I think the test (cargo) weight is 300 lb.

If rear doesn't rise with loaded cargo, then one or combination of: [1] Leaking "fill" hose located under the resevour (most likely & cheepest thing to fix), [2] Leak in any of the lines or fittings, [3] Leaking accumulator(s) (very likely $200 for 2), [4] Bad pump (PS & Self leveling in one), [5] Bad controller.

If fluid is low, there's a leak in the system. Simple test of accumulators is ... turn on engine, partially remove fill cap containing fluid return hose - if fluid is foamy, nitrogen is coming from bad accumulator(s). This cap contains a thin mesh cylindrical filter about 4" long. You must use Febi #2615 Hydraulic Fluid, and put it into the resevour. Only Power Steering fluid goes into the PS pump.

Bob