stock mechanical pumps used with the quatrajet carbs put out somewhere between 7-9psig on average. the float leverage ratio is better on the q-jets for keeping the needle on the seat at the higher pressure than the holleys and eddy's.
you generally show the highest psig at idle when the fuel volume requirement is the lowest,, and should for sure have the float bowl full and needle and seat closed up,, for the most part.
i'd check the sock in the tank or the rubber line connections on the suction side of the pump. possible that it's sucking air in at one of the hoses/connections on the suction side??? if not,, then maybe a bad pump.
did the pump sit unused for a while,,, after having fuel in it previously? could be a dry rotted diaphram with no hole in it yet??
hope it helps,
you know you really blew it when,,, all your parts will again,, fit loosely in a 5 gallon bucket!!!!