Grand Banks 46 Classic forward water tank issue

When isolating the forward water tank in our 46 GB Classic, pressure does not build up enough to produce any more than a spitting and sputtering of water from any outlet. Isolating was done using the ball-style isolation valves for the forward and aft tanks located just prior (upstream) to the junction leading to the pump. Isolating the forward tanks yields the sputter, isolating the aft tank produces a strong and steady flow. I have checked the hoses (free of restriction), replaced the in-line check valve/backflow preventer (in the correct position), ensured the vent line is clear and the Pegler valve is in working order. The only thing I have not done is remove the pipe assembly into the tank. Any other suggestions out there?