dump cylinder or pump

I have a Fruehauf quad axle dump trailer and the last stage of the cylinder you always have to rev up is it the fault of the cylinder or the pto pump? My mechanic said it just had air in th cylinder to bleed it off which we did and it worked better for a couple of times but still goes back to the same thing. It gets to the fourth stage and stops unless you rev it up even than sometimes it does not want to go.

 

The pump does not chatter. The tractor has a fifty gallon tank on it would that be enough oil for a four stage cylinder? We were just using the tractor for a lowboy and started getting into demolition more and bought an old fruehauf semi 1996 so I am not sure if the tracor is set up right the pump works fine lifting the lowboy but that is just a single stage cylinder. Should you open the tank when it is all the way up or would it be under pressure? Thanks for your help.

 

Up date the cylinder is a 5 stage not a four the wet kit is a muncie wet kit with 50 gal plastic tank with a sight glass on the side to check fluid level.