According to the AAMVA CDL Manual, what should a driver never do when operating a tank vehicle?

AAMVA CDL Manual §8.1.2, p.8-8-1

An open valve or manhole cover turns a stationary leak risk into an active spill the moment the vehicle starts moving, with liquid or vapor escaping along the entire route. This single rule is one of the most consequential checks for a tanker driver because it is the last line of defense before leaving the loading area.