John C.
Senior Member
Rotary piston hydraulic pumps do not suck oil into them like a gear or vane pump does. They do not self prime at all. It has always been my practice to crack fittings on top of the pump and put a little air pressure into the tank till I get oil out of all the fittings. Only once the bubbles clear do I start the engine. I only start the engine at low idle and let it run for a bit. Then I'll slowly start moving the functions clearing any air from them. After all that I'll set the implements down in the fluid level checking position and top off the hydraulic oil as necessary.
Lets say you have installed the pump and now are putting hydraulic oil back into the reservoir. There are many small passages inside the pump that have air inside then that takes a bunch of time to bleed out. You crack fittings at the highest parts of the pump to bleed as much air out as possible. Most of my work was done on Komatsu machines but also a few Hitachi and Link-Belts. Most of the pumps I've had apart had servo pistons inside the pump to control swash plate angle. Most use a spring on one servo piston and nothing on the other. The spring can be set up one of two ways. The spring can be on the side of that pushes the swash plate to full tilt or to perpendicular or no flow. There is supposed to be oil behind both servo pistons. In either case when you start the engine the pump is supposed put oil through the control valve block and a small supply comes back to the pump control blocks through a small hose. If there is no oil in that hose the pump goes to full stroke. It doesn't take much movement of a piston against the barrel with no oil in it to score the barrel and seize the pistons to the barrel.
That is probably the best way I can explain it.
Lets say you have installed the pump and now are putting hydraulic oil back into the reservoir. There are many small passages inside the pump that have air inside then that takes a bunch of time to bleed out. You crack fittings at the highest parts of the pump to bleed as much air out as possible. Most of my work was done on Komatsu machines but also a few Hitachi and Link-Belts. Most of the pumps I've had apart had servo pistons inside the pump to control swash plate angle. Most use a spring on one servo piston and nothing on the other. The spring can be set up one of two ways. The spring can be on the side of that pushes the swash plate to full tilt or to perpendicular or no flow. There is supposed to be oil behind both servo pistons. In either case when you start the engine the pump is supposed put oil through the control valve block and a small supply comes back to the pump control blocks through a small hose. If there is no oil in that hose the pump goes to full stroke. It doesn't take much movement of a piston against the barrel with no oil in it to score the barrel and seize the pistons to the barrel.
That is probably the best way I can explain it.