The alpha's have impellers, the Bravo's don't. When you say you loosen the cap on the heat exchanger, is that the fill cap where you put in coolant??? I'm not sure about that...??? But essentially you have a big upside down U shape with one end being the seacock intake, then up through the strainer (if you have one) through the seawater pump, through the heat exchanger, through the exhaust elbows
(the top of the upside down U), and then down through the Y pipe out the prop. Without some kind of vent at the top, it's airlockd when you drop it into the water so water can't make it up to the seawater pump to prime it. When you use a pickup on the drive it's easier for the seawater pump to prime. You could install a strainer in your raw water path (above the water line but below the top of the elbow) and prime the pump from there......
I'm at a loss for explaining why loosening the cap (if you mean the coolant fill cap) would affect it....
Unless you correct it I would expect seawater pump impeller life to be pretty short....
