The trouble is trying to diagnose the lack of pressure. The pump does not run 1500 PSI all the time, that is the upper limit at full load. Without a load the pump is simply circulating fluid at minimal pressure.
Either there is an issue in the hydroboost that is dumping all the flow into the return line so nothing is making it to the power steering. The power steering has an issue where the fluid blows through without doing work. Or the pump has the pressure relief valve stuck open. so the fluid just circulates mostly in the pump. The 150 PSI is probably just the pressure that can be built as it isn't a completely open system and there is a little backpressure somewhere.
Are you getting any flow? with the steering wheel loaded, how about now.
My guess is a relief valve in the pump is stuck open. If you deadhead the pump with a pressure gague you should see the relief valve pressure setting.