1982 280SL Throttle Pressure Question

Are you sure the transmission is controlled by vacuum and not a pressure rod or cable from the throttle linkage? Take a look at the top rear of the engine, especially its right side (looking from the front). There's probably some throttle linkage there and a rod or cable down to the right side of the transmission. That rod or cable transmits the throttle position to the transmission so the box will know when to shift.

The correct adjustment of the rod or cable's length is done by detaching its end from the throttle linkage and gently pulling it and allowing it to return to its relaxed position. That position should allow the end to be reattached to the throttle linkage WITHOUT pulling or pushing the rod or cable. Adjust the rod's length or the cable's mounting nut so its ball can be reattached to its pin without pulling or pushing the rod or cable. The transmission should now shift appropriately to the throttle's position.