Make sure you grabbed the return line and not the vent line.  Also, when you changed out the pump, was there an obvious size difference?  Did you have to spice on the wires?  My pump whined real bad, could even hear it over the obnoxious flowmaster.  Took it out to put a denso unit in ($150, packaged as another brand, but said denso on it), and found out I had a crappy walbro universal pump in there.  Half the size of the denso unit, and had to be spliced in.  Put the denso unit in, no whining, truck starts better.