first. lets start by getting you using the right terminology.
you are talking baout hydraulics on the 3pt. NOT the PTO
the PTO is the lil shaft that sticks out the back of the tractor that spins..
a lift that looses prime can be a few things.
common problems are bad orings at the plumbing manifold from the sump to the pump.
unfortunately.. equally common problems are shaft seals inside the pump... that one is a lil harder to rebuild.. etc.