85 S/Box Silverado Dual Fuel Tank Question

[hatzie]

What led him to believe the valve was at fault? If it's sucking air at the valve it will leak when it's off. If it's leaking internally you can blow 5PSI through it to check. Parts changers.

Was the LH tank full when it misbehaved?

When the RH tank reads 1/2 how many gallons does it take to fill? It's a 16 gallon tank that should take 12-15.5 to fill from empty on the gauge.

You may have crud in the tank being kicked loose by higher alcohol content fuels or just plain rust blocking the socks.
I've seen pinholes in the sender pickup tubes at around 1/2-1/4 from the tank bottom.
Pull the bed, then the senders, and take a look. Replace the rubber fuel hoses while you're down to them. If the tanks are dirty but sound you can get em cleaned and just replace the socks.

You could be sucking air around the hoses on the new switching valve that the parts changer installed. Look for wet fuel lines.