1986 Ford F700 dump hydraulic question

I've just bought a 1986 Ford F700 dump truck. There is a small hydraulic reservoir welded between the frame rails. There is a filler plug on top that is just a typical pipe plug. There is no vent anywhere in the system. The first time in a day that you lift the bed, it sounds like the pump cavitates. Sort of a rattling sound out of the pump. After the first lift, everything is fine. Subsequent lifts are smooth and quiet.

I notice that after the first lift, if I remove the filler plug,...

1986 Ford F700 dump hydraulic question