the lock-off is located between the hose from the tank and before the regulator. in your truck I think it would be in the drivers left side, near the radiator and fan, but tucked into the corner near the leg of the overhead guard.
I would strongly urge everyone to get the proper training and information before attempting any repair on any product (not just in this subject we are discussing here).
a -little- knowledge can be a very dangerous thing.
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