pressure test it. put a 0-3000 psi gauge in the plug in the option cover, chain the lift arms down.. reg it, and then run the lift up, rpms up. see what pressure it makes ont he gauge vs relief.
ps.. if it has a huge leak in the pressure / cyl side of the circuit.. that may be the issue too.. as you are bleeding off the oil as fast / faster than it 'makes' it.
that said. the vane pump usually gets changed out.. etc..