Why is it dangerous to overuse the trailer brakes (hand valve) on a tank truck?

The trailer brake hand valve (sometimes called the 'trolley valve' or 'broker brake') applies only the trailer brakes. Some drivers use it routinely to stop the trailer first, believing it prevents jackknife. This is wrong. Locking only the trailer wheels — especially on slick surfaces or while the tractor is still being driven — can cause the trailer to slide and jackknife around the tractor. The correct technique is to use the foot brake, which applies both tractor and trailer brakes proportionally as designed by the system. The trailer brake hand valve is for emergencies (testing trailer brake function during inspection, and certain specific maneuvers); it is not a primary braking control. In a tank with surge, locking only the trailer can be especially dangerous.