Follow Gregs and Layouts lead first,,since ya are stating its when a driver enters your truck and ya dont dump the air,,,the weight of these forklifts can damage internal parts on the air system by forceing air back in the wrong direction..My M-2 leaks on the secondary side also after filling the air bags as I pull off the docks,,,seems if I cycle the manual switch to dump the air a few times it seems to build and maintain proper air...I to had freightliner look into this but got no help.
As far as the leveling valve,,if ya do force it move you propely will be hearing air thats what the valve does..Maybe a release valve sticking shut???not letting the air out fast enought??
The part bout leaking when shut down and parked could be other issues beside the air ride,,If ya got shop air at the house fill tanks that way and listen for other leaks..
Well good luck without being there and climbing under the truck,,,we are all guessing for ya..
G/luck,,,hope ya dont have to pay the shop again..