7.4 TRACTOR-TRUCK AND SEMITRAILER BRAKES.
Improper operation of brakes on tractor-trailers and
semitrailers may be hazardous. Great care should be
taken to prevent accidents.
(1) Parking. When a tractor-trailer unit is parked
and the tractor air compressor is not in operation, do
not depend solely on setting the air or vacuum brakes to
secure the vehicle. The air or vacuum pressure may
leak and release the brakes. To secure the vehicle,
apply the tractor parking brake and place wheel chocks
at trailer wheels.
(2) Independent Trailer Brakes. When towing a
trailer with independently operated trailer brakes
(controlled from a separate lever), apply the trailer
brakes slightly in advance of the tractor brakes. This is
especially important, when towing a full trailer, to
prevent the trailer from "running-up" or "jack-knifing"
on the tractor when making sudden stops or when