that seems rather odd, the reason i say this is because this 'balance line' you speak of is only there to provide vacumm to the evaporator to assist the diaphragm so it can open the secondary valve in the evaporator to send fuel to the primary chamber and then on to the mixer.
By removing the vac line from the evaporator and it starting and running tells me you may have the wrong evaporator on the truck or it has some kind of malfunction.

This is just a basic generic explanation
The way that is supposed to work is when the engine is turned over it provides a vacumm from the mixer assy through that hose to the evaporator, this vacumm opens the valve inside the evaporator allowing the LP Gas to flow through it, get vaporized and then out to the mixer. The pressure from the evaporator pushes open the mixer diaphragm mixing the LP Gas with air.

When you turn on the key and power on the system (without starting the engine), and the LP lock-off opens i would listen at the mixer and see if you hear any pressure leaking into the mixer, it should not. If you do hear anything then there is a problem with the evaporator or the mixer diaphragm. One other test is to remove the hose at the mixer where the LP fuel enters and then turn the system on and try to start the engine and see what comes out, it should just be a vapor rushing out. If you get more than vapor then its an evaporator issue, if you get nothing then its still an evaporator issue or possibly a lock-off issue.