I replaced the fill neck and cap. in Cali they pressurize the system to about 5psi and it needs to hold. So the LCV is needed to hold back the pressure so it dose not vent.
Actually, that's not quite how it works. The way they should be testing the evap system is to pinch closed the vapor line up at the charcoal canister (where it is easily accessible). The LCV won't hold back pressure unless it is closed (gas has filled it enough to lift the float to the closed position). It isn't a vapor check valve, just liquid. So they close the line in the front to pressure test via the connector that fits over the filler hole. Besides something like a bad o-ring on the valve (and possibly the roll-over valve as well) you probably also want to look at the condition of your hoses. Mine were shot and replacing them solved my leak test fail. Also, pressurizing my system was actually forcing fuel out through the fuel line (which was also cracked).
To make sure mine passed after fixing things, I made my own pressure testing setup before taking it in for the re-test: http://jeep.smallcraft.net/evap/