With an Automatic you just use a pump that is self contained. (just like the ones on lift gates,fork lifts, and Suspension set ups) It usually runs a 12v motor on the end of the pump to drive it.
The way I have seen it would be a hydraulic pump that pressurized 1 single line to go up and then hit the switch the other way and let the weight of the bed bring it back down. (1 pump, 1 dump, 1 3 way switch, 2 hydraulic lines) You can either buy these pumps with a number of different gears in the pump head or you can use one from a lift truck. depending on which you use make sure the cyl and hoses match the pressure it puts out.
I have seen 1000 psi on lift gate pumps, 2-3000 on suspension pumps, Custom pumps and some oddball ones I have seen as much as 6000.
Hope that helps..??