If you can bleed it, and it works good for awhile, then you're getting air in the system. That's usually not the pump!
Check the line between your reservoir and pump. Your pump could be sucking air in from somewhere. Could be bad seals
on the pump fittings.
To check the pump hold it hard-over (port or stbd) against the lock and see if it bleeds down (the wheel
turns), and if your not losing fluid and the wheel turns then you either have a bypass in the cylinder or pump. Cap the lines to the cylinder and try again. If it by passes then maybe the pump.
Do you have a pressure relief valve in the system?