A little more explanation: When the engine is running, the fluid only circulates between the reservoir, pump, and SLS control valve when the car is static (no change in the load). The fluid on the strut/accumulator side of the circuit is not moving anywhere, so you need to somehow get to that side of the system. If you fully load the rear of the car, and then start the engine to pump out the old fluid, the SLS control valve will simultaneously be trying to raise the rear axle height back to normal, and so a greater than normal volume of the fluid will be moved to the strut/accumulator side of the circuit. Bouncing on the rear bumper (instead of a load in the rear) would help move a bit of the old fluid out, but you really need to bleed out the old fluid using the bleed valve or by loosening the lines at the struts.

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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