When I was maintaining similar dump systems, we used a plastic squeeze bottle with a U-shaped piece of tubing in it, and filled the cylinder "blind" with the bed down until the level reached the fill hole. You can also lower the bed onto a piece of railroad tie across the frame rails to get better access to the plug. Then we would cycle the bed up and down a couple of times with the fill plug removed to purge any excess fluid. BTW, that fill plug should have a vent hole drilled in it. The fluid reservoir above the piston operates at atmospheric pressure.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!