Yes, over-pressure in the delivery system can FORCE the needle valve open.
One of the four fuel bafel nipples is the breath VENT line. Goes to a tee or Y fitting
that branches, one going up to a one-way valve to allow air IN, up to thru-hull to a breath bugle fitting. Other line goes thru a OUT one-way valve and vents over-pressure thru-hull under the rub-rail. Both valves are the same, except oriented in-
line directionally. Unless clogged, open at 3-5PSI. Your Pop-off Tester is very handy
to check the one-way valves operation. I have used my air-chuck, lightly. Bills86e
You should have a small expell when removeing gas cap. It is a pressureized system.