Suction blockage comes in two types:  dynamic blockage and static blockage.  A dynamic blockage only obstructs the suction line when the pump is trying to suck fluid, not when the pump is turned off.  This on-off quality makes the dynamic blockage very difficult to diagnose, since when you take the suction line off the pump to drain it into a bucket, fluid pours out, but when you run the pump, the fluid flow dries up, the hydraulics don't work, and, of course, the pump is immediately ruined from being run dry.

Semi-liquid sludge around a suction strainer in the bottom of the reservoir can create a dynamic blockage.  The sludge floats freely slightly away from the strainer in the bottom of the reservoir when there's no suction, then the second you turn on the pump -- wham! -- all of the sludge gets held fast against the strainer, blocking all fluid flow to the pump.  An imploded suction hose, where the inner layer of rubber inside the hose closes off under suction and reopens under no suction can also create dynamic blockage.

Static blockage, on the other hand, involves a simple closing off of the line at all times.  For example, a foreign object or valve part can obstruct the line.  When you take the suction hose loose, only a trickle of fluid comes out of the hose.

Solve the problem by taking apart every inch of the suction line from the reservoir to the pump.  Find the blockage and remove it.  If you insist on having a hydraulic filter or strainer, put it in the return line from the valve body to the reservoir, not in the suction line.