Like Skunk said, the pipe is the choke point in the bilge. The bulkhead forward of the tank under the cuddy entrance is probably acting like a dam. If you can get to the back of the pipe from a hatch behind the tank and run something through it it may break loose the guk in there and allow some draining. When I replaced my tank I ditched the pipe and foam around and under the tank and my bilge flows freely.