Are tank selector valves known to block fuel flow?

[chevymotocross]

Hey guys, my 87 Chevy 305 TBI has low fuel pressure. I am only getting 8 psi at the TBI inlet. I recently bought an adjustable fuel regulator to pump up the pressure a bit for performance. I installed a gauge before installing to get a baseline reading. Only 8 psi. I then installed the adjustable regulator, hoping to bump it up, but with it cranked all the way, it is still at only 8 psi (factory should be 9-13).

I changed the fuel filter with no luck. I tested for a bad regulator by pinching off the return line and seeing if the pressure rises. It did not rise so the regulator is fine.

I don't see any leaks or kinks in the lines.

Bad fuel pump? Well I would say absolutely but the tank and pump are only 2 years old (Carter pump). My truck has dual tanks. The other tank leaks, so I don't use it. For testing purposes, I put some gas in it, selected that tank, but pressure is still only 8 psi. How is it possible that both pumps are putting out the same pressure?

The selector valve works, it switches between both tanks fine but is it possible that it might be partially blocking flow (due to age)?

What else should I do? Truck runs fine, and gets decent mileage (15mpg), but I feel there is something wrong with the below factory rated pressure. Plus, I paid $80 for an adjustable regulator!