Check the bottom surfaces of your mounted 10 micron fuel/water filters with a photo. If they look like this, it's really easy for them to have an air leak, even if you feel like you've screwed the filters on tight. The corrosion makes it difficult to gain an adequate seal.
I also had this issue and also went so far as to pull the deck plate and replace the 4 anti-siphon valves on the tank pickups. But these canisters is your primary opportunity for an air leak between the selectors and the primer bulbs, and it's where I was pulling air with motors of a similar vintage. Best of luck!
Sharkbait, this is great and I am leaning toward this potential problem because my oil/fuel seperator housing looks just like yours and I am pretty sure it has to be a small air leak somewhere between the fuel selector valve and motor. I was thinking of replacing the primer bulbs next and then fuel lines between selector valve and tank. Now i may start with these housings. Where did you buy the housings and were they expensive? I may try cleaning but mine are pretty corroded.
Well I replaced the fuel/separator housing thinking that was a real solution and it didn't do it. Interesting, I had the problem at the dock with the engine losing it's prime and shutting off so i went grabbed a fuel caddy and hooked a line directly from the portable tank to the engine bypassing all the fuel lines and the engine started and ran like a kitten. I thought okay we have definetly isolated it to the tank and lines for air leak. I am still going with that but as soon as i re connected the fuel line back to the motor from my existing lines, the engine started and ran liked a kitten as if there was never an issue. I am still going with problem in the fuel lines from engine to bottom of the tank but i thought that was interesting and i guess i still can't rule out the low pressure fuel pump as maybe it was just an intermittent failure and after the portable tank primed the engine it just picked up and continued to run with the old lines re-connected. PUZZLING!