Thank you everyone for the help!
The problem was infact the return pump. As we were rebuilding the fuel pump, we found a large piece of rubber debris inside, nearly blocking off the return side of the pump. We replace all the gaskets and darts. The darts were pretty difficult to replace. I ended up using a small pick that I rounded out with a file to make it as blunt as possible in order to push in the new darts. Even with the tool it was still difficult. I still had to lube them with oil and press down on the pick while I worked around the edges with a small flat head.
