Abstract: Two forms of device are described for preventing unintentional removal of the contents of mobile tanks before unlocking the vent caps thereof. In one form, a guard unit is provided which is movable between positions permitting or interfering with attachment of a drain conduit to the drain opening. A guard unlocking linkage is provided which includes portions movable between first and second positions respectively, with the vent opening cover being required to be opened before the unlocking linkage can be moved to the second position. When the cover is open and the unlocking linkage can be moved, the guard unit is moved to the position permitting attachment of the drain conduit.In another form, a locking plate is provided on the vent cover and a notch on the plate registers with a locking device connected to a first hydraulic piston and cylinder assembly.