1941 D2 5J Fuel Transfer Pump Pressure Problem.

Could well be Patrick. Unlikely that your transfer pump would have worn out overnight so go ahead and clean the bypass valve again. I plumbed a cheap filter from Tractor Supply into my line just before the tp to catch any crud. That said, my gauge registers nothing while starting, and barely gets into the white / caution while running, and it seems to run fine. Of course, the gauge could be faulty but when bleeding, there doesn't seem like much more than a few psi coming out even while running.
Is your tank full? I found mine was harder to prime when the tank was down. If this is suspected and you can't fill it, apply 5 psi to the tank filler opening and prime/start.

It’s been a few years since I bought one for a pre 1960 engine but seems like some of the poppets had a rubber seal on them instead of being a hard plastic like the later ones are. I remember that once the rubber got stiff, they didn’t seal well.

If no rubber sealing surface on the poppet, any erosion in the seat surface will result in worse leakage. You may need a new poppet and or carefully resurfacing the seat area.

The fuel tank is full as of a couple of weeks ago but even when it was not it acted much the same way.

Hi Team,
the hard plastic relief valve poppets were problematic back in the day and were a known trouble source.
A piece of crud--dirt, lint, hair etc. held them open and if left long enough erosion occurred at the crud site and gave low pressure issues.
Rust pits on the pump body valve seat area required facing on emery paper, filing in the field and emerying or machining in the Dealer shop if too deep for a quick touch up.

I used to wad punch a suitable sized hole in a piece of fine emery or wet/dry emery paper to put the poppet guide end thru and resurfaced the poppet valve face ring area ---from new the poppet sealing ring area is actually rounded so it has a line contact. Afterwards you need to reduce the area of the face carefully with a fine warding file, or else the surface area is larger and so the surface pressure is less and lower fuel pressure results- stretch the spring a tad and see what you have.
Sometimes a quick touch on the emery will show an unseen worn, indented spot on the poppet seat area.

Cheers,
Eddie B

Thanks all. It looks like I'll be pulling the pump off again.

My gauge on my 12 never shows good pressure when cranking with the pony. Pump was near new. 0 wear. I had already stretched the spring. Ordered the poppet without asking the price. Couldn't be much. $48.00 and didn't change a bit. Live with it. I do have a 1/2 inch, in line screen filter that helps with crud. Hose clamp style.