Accumulator help

[WANNA930]

I have a nother car with CIS but it is almost identical and you Porsche guys are smarter at this CIS stuff.

I sort of understand accumulator but have a question. Car has sat for days. Go to change filter. Fuel pours out but does not stop? Remove filter now I confirm fuel still flowing. Pinch off tank to pump hose stops flow.

Valve in pump stops backflow away from filter?
ACC if not working wouldn't have pressure and not for that long?
Could somehow fuel be sucked through acc back to tank line??

Actually that is where I get a bit confused with 3 port. When pressure between in and out port gets too high it is bleed of through 3rd back to tank correct???

My system lays flat with tank being higher and vertical, could I just be sucking gas through atmospheric pressure?