I don't see the pump in your drawings. If you go back to post #7 and look at ref #B that should be the location on your power shift pump. At least I think so.
Are you going to test the other port? I would check everything humanly possible in the cover area. Any other relief valves, etc that could be pulled and checked for spring breakage, clogged ports, etc. Pulling that power shift pump is going to be major. It would be a terrible feeling to pull it and find it wasn't the real problem. I know it is possible and has probably happened but I have never read of one of these power shift pumps failing. I have read of clogged ports, broken springs, stuck valves.
If, just if, a relief valve spring is broken, or relief valve stuck, it could loose pressure. Ok, I preached long enough, just doubtful the pump is the problem. Sure can't know.