I just skimmed the thread. But my question for you is do you have sea strainers? If you try and flush the boat with muffs the impeller needs to try and pull the water ALL THE WAY Up the drive with only air in the pipes. Not good. If you had sea strainers you at least get a pot of water to start with. In fact I just changed my friend pump this morning. He would normally flush the boat in barrels . Never a problem because the water was to the top of the drive. Somebody told him to just use the nose cone flush.... he tried it once lol. There was nothing left of the impeller.

So I'm going through sea water pump issues right now with my boat and I thought I'd re resurrect this thread it's a great thread and has a lot of good information on it.
as far as commenting on loosening the intake hose at the water pump that will not do it. the hose runs downhill from the transom so the air that is trapped in the hose needs to go up. if the hose was cut open at the highest point, and a pipe with a petcock was installed that would bleed off the air lock in the hose because the air will travel upwards and bleed out. I think this is a good solution for people that r loosing prime. It would be very easy to do at the launch before you fire up the engines. Open two petcocks until water comes out close them off and fire the motors up.