When should a CMV driver use the trailer hand valve (trolley valve)?

The TRAILER HAND VALVE (also called the TROLLEY VALVE or INDEPENDENT TRAILER BRAKE) is located on the steering column or dashboard and applies ONLY the trailer brakes when activated. CORRECT USE — PRE-TRIP INSPECTION ONLY: The hand valve is properly used to TEST trailer brakes during the pre-trip inspection (pull gently to ensure it creates resistance, then release). WHAT IT SHOULD NOT BE USED FOR: (1) Parking: Using the hand valve as a parking brake is dangerous — it doesn't lock (spring brakes do); releasing the valve allows the trailer to roll; (2) Driving on slippery roads: Applying only trailer brakes on a slippery surface while the tractor brakes are not applied can fold the trailer at the fifth wheel — a textbook jackknife cause; (3) Regular service braking: The foot service brake applies all brakes simultaneously with proportional force — this is the correct brake for all stops; (4) Helping stop on grades: The instinct to 'grab the trailer brake' when descending is dangerous — use engine braking and foot brake. WHY HAND VALVE CAUSES JACKKNIFE: When you apply only trailer brakes, the trailer decelerates faster than the tractor (which is still rolling freely forward without brake force). The articulation point (fifth wheel/kingpin) becomes the pivot point for the trailer to swing sideways relative to the tractor.