Hydraulics fundamental

Pressure moves the load, flow dictates how fast that load is moved. A two hose system as you reference uses flow/pressure to both extend, and retract the cylinder. A vented plug will allow gravity to drop the raised bed in theory, but not always the case once wear comes into the picture and the bed can "hang" in the air. This all depends upon the design and included angles of the design but usually hydraulic systems that are "double acting" are in place for a reason. The hydraulic down pressure also controls decent rate, and more/less holds the bed down when not in operation.

My unsolicited opinion is you would be much better off spending two to three hundred bucks on a double acting hydraulic pumping unit mounted and plumbed into the system.

Unless someone built it with old-style high-pressure/low volume implement cylinders, I concur with all the above, with the following notes:

What you PROBABLY have, is a double-acting cylinder on the trailer.  One hose goes to the port which extends the cylinder.  The other hose goes to the 'back' side of the piston, it retracts the cylinder.

You CAN install a vent on the back side of the piston (retract) and just use the front side as a single-acting cylinder (gravity down).  My dump trailer's electrohydraulic unit does this, and the cylinder has a vent directly ON the cylinder.

In doing this you'll need a significant volume of hydraulic fluid available in your tractor's pump, and due to your pump's design (high pressure/low volume), you'll be screaming the engine and waiting a long time for that cylinder.

IF, instead of venting, you connect that back-side of the cylinder to your tractor's reservoir, and cycle the cylinder a few times, adding fluid to the reservoir, the back-side of the cylinder will become additional fluid volume to compensate for your tractor's small lift displacement capacity.  Doing so will NOT give you down-force, it'll just take up the extra fluid you'll need.  Of course, if your hose couplers don't seal, this will immediately all flow out on the ground when you disconnect the lines... so you'll want to couple them together or cap them.

And it'll be slow... VERY slow... but It will generate significant force ...And as others noted, the cylinder will be a risk of blowing seals IF you ever 'top out' the cylinder (stall it at end of travel) or if you overload the dump box.

As for 'return', I have had circumstances where MY dump trailer did not want to retract... just a little too light, and the cylinder packing was a bit too snug, and oil was a bit too cold, it took some fussing to get it to come back down.  At that point, I DID wish for a double-acting setup... but it doesn't happen often.

IF I was building a dump trailer specifically for use behind a tractor, then I'd set it up to run off the tractor's auxiliaries AND use it for extend and retract.  I use my current dump trailer behind my D17 all the time, but I pull it on the road with my pickup too, so the electric unit works just fine.  I can operate the trailer dump from tractor seat using the pendant control, it's got a 12ft tether, I just hang it on the axle tube.  Uses a common car battery, and I have a little solar cell atop the battery box, with an inexpensve MPPT charge controller in there...  If I'm running it hard, I'll connect a charger to it at night, but during summer it takes care of itself...