Case 644 hydraulic pump will not prime / make pressure.

I recently rebuilt the Koehler 14HP engine, but in doing so, I had to remove the hydraulic pump, reservoir and about 4 hard lines near the inner firewall for the bucket Rams. The good news is that the engine rebuild was great (engine runs better than ever) but the hydraulics will not work. The tractor will not move, and none of the Rams front or rear will even make the slightest movement. I have ran the engine twice for several minutes at low, medium and full throttle while operating all of the controls without success. I do notice that the hydraulic fluid (I'm using a 15-40w engine oil) seems to be gurgling and slightly bubbling a little bit when I watch the reservoir with the fill cap open. The flow rate seems to be fine from what I can tell, but it's definitely not building pressure. I replaced the pump about 10 years ago with a new one, and it was working fine prior to rebuilding the engine.
I was thinking that the air in some of the lines needed to be bled out, but was hoping that someone could steer me in the right direction.
The tractor has the front bucket, rear hydraulic 3 point, hydraulic pto port on the right rear fender for my hydraulic rototiller and another one below it for the lawn mower.
Thanks!

the oil is gravity fed tot he pump so it should bleed out as soon as the pump was turning,, did you by chance leave a plug/rag ect in the suction line when you hooked it back up? only other thing I can think of is maybe a key fell out of the pump drive when installing them and the pump is not turning
just throwing out ideas
cnt

Pumps on upside down.

I'm pretty positive that there aren't any rags or pieces in the lines. I can see the oil flowing in the reservoir when it's running. As far as the key is concerned, I'm absolutely positive that is installed as well. Still puzzled as to what is (or isn't) going on.

I'll have to take a look at that tonight or tomorrow. There are two pieces to my pump. The block (with the impellers inside) and the back piece that has the inlet and outlet pipes on it. I'm assuming i just need to rotate the large part (with the impellers inside) 180 degrees. Gonna be a big mess with the oil again, but if you're pretty certain that's the problem, then I'll try it for sure.

If you didn't have the pump apart while you built the engine don't take it apart now. Can the pump inlet and outlet lines be interchanged if the pump is rotated 180 degrees. I am pretty certain that is an open center hydraulic system so it would bleed immediately upon start up. Providing the pump is moving fluid as you say. HTH Tom