ok a few things i would check,
1) sure, check the filter and the check valve on it and also the sump filter/screen in the tank and the pickup tube.
2) pull the pump off and check the spline on the shaft.(might do this one last)
3) look on the hydraulic control valve, it has a relief valve located on one of the end housings, has a dome nut and another adjuster nut, pull that and make sure it is good, no broken springs or corrosion.
And then there is a priority valve inline between the control valve and the pump. That valve shuttles fluid between the hydraulics/steering, pull the spool and make sure it is not stuck or corroded.

also while checking those, look at the hoses, especially the suction side and make sure it is not collapsed.