Originally Posted by barry12345

That is the old style primer pump and they are notorious for your types of issues. The one you want has a black cap and you do not unscrew the top to use it. I am not suggesting it is all your problem. It could be though just as well. It is a piston in a bore basically . It uses the check valves in the lift pump to function as a pump. If exercising it with the nose submerged and bubbles are appearing. It is leaking by the internal seal of course. My quick test in place when suspect was to stretch a small balloon over it. Squeeze as much air out as possible. On engines that were still running but had some apparent air issues. In a short period the air left in there was consumed if it was bad and the engine smoothed out.

After twenty thirty or forty years I just suspected the seal in some of them has gotten tired. Or compromised by the diesel fuel.