So when I take the low pressure line off at the back of the fuel pump and blow into it I head bubbles then when I take my mouth off I get a gush of fuel out (catching it Into a bucket) and then something seals off inside the tank and I get no flow?

does anybody actually know what mechanism is inside the tank? Somebody on here must? why would it behave like that

it’s not a blocked breather because nothing comes off even with the filler cap off