Thanks for the helpful information about the leveling problems with my 87 300TD. With that new information, I was able to read the numerous posts regarding self leveling troubles and get much more up to speed.

When I do the bounce test described above, there is give in the springs and struts, leading me to believe that the accumulators are not the chief culprit (though I may replace them anyway). Is that a valid conclusion?

I added weight to the rear end. The self-leveling did not work, and the rear end just sagged. This leads me to believe that the issue is either the tandem pump, the level controller (including the linkage, as mentioned above) or the (ouch $$) struts.

I visually inspected the components. Everything looks okay, except that the hydraulic struts look worn, mainly due to the fact that the rubber bellow around each is dry-rotted and broken up. Does that necessarily mean they are dead? If not, can/should the rubber bellow be replaced?

I released the pressure in the system via the nipple on the top of the controller. Surprisingly, the rear end dropped, suggesting that the system maintains pressure.

Next, I plan to test the oil pump and controller. Does anyone know what threading the nipple is on top of the controller (it looks like 3/16 brake line fitting, but I suspect it is M10 1.0 or something close to that). I understand that the pressure at the bleed fitting on the controller should be at least 133 bar when the car is on and the level controller is in the "filling" position (up). What should the pressure be right after the controller is switched to the "emptying" position?

If pressure checks out okay, I will bleed and re-pressurize the system, check the controller orientation and linkage, as suggested above, top up the oil and visually inspect the hydraulic struts for leakage. Beyond that, are there any other suggestions for confirming whether or not the hydraulic struts need to be replaced (at a cost of $1100 plus labor)?

Thanks