Heavy: The way you describe it the pump would cavitate and start to pump air. It works because the upper end is connected directly back to the suction side of the pump as I first said. A simple bypass system. So it really doesn't run out of oil it simply circulates and yes that is a bit of genius. Works the same with or w/o a reservoir. IF is is newer and has a relief valve built into the directional valve then it goes directly back to the tank. The only old underbodies I've seen with reservoirs were units built to have a multi-function hydraulic system and used a bank valve to operate various other functions like a plow truck, etc. and cooling became an issue. They just weren't needed. I'm curious as to what type of control is on that firewall mounted valve and what type of valve it is.
Also the liftgates of today utilize DC pumps and electric controls in 98% of applications. They use a relief valve and have no mechanical linkage but I am familiar with the old ones too as orchidskip describes. Almost never see one anymore or even a new gate with a PTO pump. I haven't sold one in 10+ years. All this stuff has evolved of course but it has not necessarily improved. In 50 years you won't see any electric pumps still running I'd wager.
