Impeller vanes can take a set and don't press well enough against the pump body. Also old impellers develop tiny cracks in the tip of the vanes which let air past. So replace the impeller first.
Also check that the suction hose from the seacock to the strainer isn't collapsing due to old age and that the seacock or its external strainer isn't plugged. Make sure that the injection elbow
isn't plugged. Take off the hose and poke inside it with a screw driver. If you get a bunch of scale, replace the elbow
When all of that is ok and if it still won't prime then remove the discharge hose to the heat exchanger
, run the engine until water comes out, connect the hose. Sometimes (with an old impeller) you even have to remove the hose on the outlet of the heat exchanger
and let water blow out there before you reconnect it to the vented loop.