I hooked up the rollover valve and liquid check valve per the AMC FSM and the GM charcoal canister to ported vacuum.
Apparently the GM pickups have a liquid check valve that fits on the fuel sender assembly on the fuel side, and only has one port straight to the fuel vent. Using common sense and logic, and that sometimes gets me in trouble, I think the liquid check valve is similar to the AMC liquid check valve, a simple float ball that closes off the vent when the fuel is topped off. As I stated in the liquid check valve thread, I closed the vapor bypass in the liquid check valve because the diaphragm was not working and would have let fuel into the tank vent line when I topped off the tank.
I am happy to have the charcoal canister hooked up because I would smell a heavy fuel odor when I topped off the tank, I think the fuel was making it all the way to the front and the vent line was completely open below the master cylinder.
I have the Haynes manual for 88-00 Chevy pickups and it does not cover in any detail the vapor system so I am winging it for now.
1978 Cherokee, 360/T400/QT...5.3/4L60E/NP241C in the works...