The small line is the pressure side and it goes to the condenser coil in front if the radiator. The large line, the suction line goes from the evaporator coil inside the cabin to the compressor. If you charge it, it has to be from this line while the compressor is running. There is a little green sight glass on the canister on your frame. You want to charge that until the little bubbles you see going past it are very few. If you do not have any it will be over charged