Just throwing this out there, you have checked, flushed, and replaced all kinds of stuff and pressure is still to high. Swapped sensors and problem stays. Do you think it might be the gauge that is in error?

With out the direct water intake pressure is LOW, not high. I assume it's high with the transom pick up due to water is forced into it harder then with the outdrive intake. I have vessel view, the read outs are right. Temp does creeps up a bit to 160-165 may be 170 but I don't run it long enough, just short test runs. There is GOT to be an air leak and sucking air, but there is only a few places this can happen. I wanna cry.
:(

HyFive578, what kind of water pressure are you seeing on your 525's?

Man, I know what you're going through. I've been chasing same problem for 2 years - as I mentioned earlier, Eddie did some things to my drive to help, and hopefully later this month I'll be able to confirm his work.

One of the things that I continued to question myself was an air leak at the barbed fitting on the transom. Both of my engines have 2 hose clamps there, and while difficult to get to, I believe that mine are tight - but that hose is very sturdy, so I was never confident that they were actually providing a air-leak free seal.

Have you reconfirmed those clamps?

Other than the engine differences, your troubleshooting path sounds exactly like mine - and I never got to the bottom of it. Eddie said my blocks were full of river crud, so maybe that was it (sea water cooled engine). But when pulling the block drains, water drained freely.

I know you've had sea pumps off/on/replaced, but are you certain that they are mounted squarely to the mounting base and not sucking air?

Easy... I swapped the drives lines once already. Sucking from port drive to strbd, problem remained. Port pressure was good. meaning; lines, hoses, clamps etc is ok. IF there would be a problem then port would have low readings. Closed cooling, there are only a few hoses etc, that could have any problems. Hoses are good. I doubt there is a restriction, otherwise I would read "high' WP. I think the volume is less then it should be, sucking air. No clue where. Impeller and housing is ok. I might put some soap/water in the lines, plug them somewhere and air pressure. Watch for bubbles.

Any chance of the belt slipping, tensioner issue while pulling through the drive?

That could cause the issue. Not sure how to check, but belt is tight, pulley and everything seems ok, no "slipping" noise at all. No wear or rubbing on the belt or pulley.

Just a thought as with the external pick up its being pushed through the pump, maybe try swapping belts.

It's a good thought. I can do the belt swap, easy enough. The only tube that I could have any air leak is the hose from the drive transom fitting to the pump. All the other hoses are in positive pressure. No water in the bilge. Swapping the drives might moved the hose to a position that a pin hole might be closed, and that's why the pressure was good on port. But why strb. remained low? I will remove that hose if I can and replace JIC.

No, I was referring to the bypass pushing water much harder, therefore the pump and pulley aren't working as hard.