'80 gmc pick-up, fuel tank selector valve

[t359]

I just replaced the fuel selector valve on my '80 gmc sierra grande 15 pick-up, believing it was faulty. The same problem re-occurs...No fuel being allowed through the valve up to fuel pump. I have run a continuity test on all the wires and can also read 12v at both tanks. Seemed to be running fine, until I switched on the A/C. The fuel delivery then stopped and the engine shuts down. The same thing occurred last summer, leading me to believe that, after checking my wires, the valve was the problem. I have re-routed my fuel line hose for direct delivery from the auxiliary tank and it runs fine. I know I'm missing something here, but scratching my head to figure this one out. Service manual diagrams aren't much help. I notice when driving at night, my parking brake light glows very dimly instead of going out completely. Lights up bright when I start up, then goes out. Could the printed circuit be tied into this fuel selector valve problem? Thanks for anys suggestions.