If it has a pto driven pump that runs a hydraulic motor, chances are it is a high pressure, lower volume pump.  As I understand it, a dump trailer usually has a pump with somewhat lower pressure but higher volume.  The best thing to do is to get the pump number/specs - rpm/psi/gpm; then someone on here can probably line you up with the calculations you need to make.

In lew of specs, how many gallon hydraulic tank does it have?  My tank for running the belt trailer is 4-5 gallons; a dump trailer usually requires considerably more gallons to fill the lift cylinder.  If you have a small tank, you probably are set up with the wrong pump also.