During a pre-trip inspection of any tank vehicle, what is the single most important item to check?

AAMVA CDL Manual §8.1.1, p.8-8-1

A leaking tank can release hazardous or flammable cargo onto the roadway, creating a fire, explosion, or environmental hazard before the vehicle even starts moving. Because the consequences of a missed leak are so severe, the CDL knowledge test treats this as the top priority check on any tank vehicle, ahead of other routine inspection items.