https://www.truck-manuals.net/mack/ 150 OPERATION UNCOUPLING A TRAILER 1. Park the tractor/trailer combination on a level surface. Make sure the parking surface will support the weight of the trailer, and make sure that the tractor is lined up straight with the trailer. Pulling out from under a trailer at an angle may damage the landing gear. 2. Pull the trailer air supply valve (red octagonal knob) out to apply the trailer parking brakes. 3. Back the tractor slightly to ease pressure on the fifth wheel jaws. Apply the tractor parking brakes while still backing against the king pin so the combination will be held with pressure off the fifth wheel jaws. 4. Block the trailer wheels to prevent the trailer from moving. 5. Lower the trailer landing gear until the gear makes firm contact with the ground. 6. Disconnect the glad hands and the trailer electrical connector. Connect the glad hands to the dummy couplers at the back of the tractor and install the trailer electrical connector into the receptacle provided at the back of the cab.
CAUTION Make sure the trailer air lines and electrical cord are properly suspended so they will not become tangled or damaged while the tractor is being driven.
WARNING Keep legs and feet clear of the tractor rear wheels to avoid serious injury should the vehicle move when the fifth wheel is unlocked.
7. Unlock the fifth wheel. 8. Pull the tractor ahead slightly (approximately twelve inches) so that the king pin disengages the fifth wheel jaws. 9. If equipped with an air suspension, use the air control switch to exhaust the air from the suspension air bags. 10. Pull the tractor ahead until the fifth wheel is out from under the trailer. Stop the tractor with frame under the trailer. This prevents the trailer from falling should the landing gear collapse or sink into the ground. 11. Place the transmission in neutral and apply the parking brakes.
