No problem.
So I opened up my spare one to take a closer look and figured some things out.
1) As you would expect from the arrows on the body, there's a hole directly from the inlet pipe towards the big pumping part, skipping the triangle diaphragm entirely. So yeah, looks like you're right and it's not there to stop the incoming flow. The valve in the pump also opens by pressure and allows flow. At least on this particular pump.
2) The fuel wasn't leaking past my finger or the plunger from the triangle. The fuel was making a full pass through the entire pump, and then returning via the bypass hole into the triangle. On my 1st picture with the blue circle, you can see it on the left of the circle.
3) So I'm thinking the triangle bit is some sort of bypass. I'm not yet sure what way though.
It might be, that with a full tank, the fuel pressure pushes open the plunger and bypasses the pump. Although I'm not entirely sure what the benefit here is?
Or the other way around, if the pressure on the outlet pipe is too big (full carb), it goes via the bypass and back into the pump when the plunger opens.
Again, not sure why because there plenty pulse-pumps that don't have this mechanism and they cope just fine.
Might get a syringer later and do some more testing.....
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