Actually, I've been able to disassemble the fuel valve in my RV-7A with half full fuel tanks without having fuel leake out of the valve body. Be sure to inspect the stem "O" rings, and lubricate them with Fuel Lube when you reassemble. If air can get past the stem "O" ring, you'll get air bubbles in your fuel flow, and definately have more vapor lock issues. Applying a thin coating of fuel lube onto the selector part will result in a nice easy moving fuel selector when you're done.... But be careful that you done block the fuel selector ports with fuel lube...
