Under 49 CFR 180.407, how often must most DOT specification cargo tanks undergo a periodic pressure (hydrostatic or pneumatic) retest?

49 CFR 180.407(c)

A tank that looks fine from the outside can still have developed internal weaknesses from years of pressure cycling and the chemical effects of what it hauls. The 5-year retest is the federal backstop that catches that kind of hidden deterioration before it causes a rupture or spill on the road.