I will say this: From time to time I rent a local dump trailer, and I have the same experience when I let the box down from fully extended. When I mentioned it, they said to let it down and stop from time to time to slow the hoist retraction process. Hydraulic fluid level was at the recommended level before all this happened for me. I believe it is a design flaw by many to provide the absolute minimum to make it work. If I had a choice I'd make sure there is enough hydraulic fluid, and then if necessary, units like that and like the dump trailer I rent from time to time, should have been designed with a larger reservoir to handle the flow of hydraulic fluid without running over when the cylinder retracts quickly.
A hydraulic system would have an orifice in the return to control the retraction speed of the cylinder when the lift or dump bed is being lowered. If the orifice is too large, descent rate has to be controlled by feathering the valve, which is less than ideal.
A reservoir is sized to contain enough fluid to fully extend the Rams. Additional fluid beyond that serves no purpose, other than perhaps giving a little buffer to provide a margin if there are minor leaks.
You may want to look into the quality of the hydraulic fluid you are using. Foaming is a possible issue with low quality fluid. Foam takes up reservoir volume with aerated fluid, which takes more room.
I think others have called it correctly. Overfilling or plugged air vents, although a plugged vent might just result in a ballooned tank..
These are pretty simple systems. Pump, reservoir, relief valve, control valve, orifice, hoses, and ram.
