There is no way that the fluid return will lead the fluid output enough to explain the tank puking fluid. If the pump could not keep up to what the cylinder demands on the lowering side of the hydraulic circuit, there would be a vacuum created which would draw the fluid from the tank thru the pump into the cylinder. This vacuum would also draw air past the cylinder packings into the cylinder. These seals are designed to keep fluid in, not air out. Believe it or not, air is easily introduced into cylinders in a case like this.
The volume in the rod end of a cylinder is roughly half that of the base end because of the volume of the rod.
Here is what baffles me with the tech guys answer. In a double acting cylinder such as this, When you raise, more fluid is required (than lowering) because you are filling the bottom of the cylinder, the rod is in the top end. The rod takes up volume in the top of the cylinder which the bottom of the cylinder doesn't have. So, it does take more fluid to raise than to lower. If you are running out of fluid to raise, this tells me that the tank volume is way too small. Engineering snafu. So, when lowering, if the bottom of the cylinder pushes more fluid back to tank than the top requires, explain to me how the top can't fill up enough to keep up with the bottom? The pump should have more than enough flow to fill the top of the cylinder as the bottom empties. Except for a poorly engineered system.
Because of the weight of the dump unit on the cylinder, there is no pressure required whatsoever to lower this unit, only flow. If you cut the hydraulic line when the trailer is raised, it would drop like a brick. So, battery voltage would be a non-issue for slow lower. 12.5v sounds a little low for a resting, fully charged battery, but 11v under load isn't real bad. These pump units are traditionally slow. It's not pressure, but volume which controls the lifting speed, so a pressure gauge may not be much help. If you had low pressure, it wouldn't raise. Pressure is power, flow is speed. You need a flow gauge. These units have small pumps in them, not much you can do.
Total tank capacity about a gallon? If your cylinder is anything close to wushaw's, it takes way more than that to fill it for lift.
As far as your cylinder is concerned, lift the unit and make a mark on it somewhere for height. Come back in about 5 minutes and measure movement. You are checking for cylinder drift, a.k.a. internal seal leakage. No or little movement, o.k. Lots of movement, bad seals internally. You can also lift the unit and remove the line for lowering. There should be minimal flow as any flow is fluid leaking past the seals. If you do this, please be very careful as removing the wrong line can kill you.
This just sounds like a poorly engineered unit.
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