WAC 296-307-07007
What requirements apply to motor
vehicle brakes? (1) You must ensure that motor vehicles have
brakes that will safely hold the maximum load on maximum
grades.
(2) Trucks parked on an incline must have the steered
wheels turned into the curb and must have at least one
"driver" wheel chocked on each side, independent of the
braking system.
Exception:
If the truck has a functioning secondary braking system, the turned wheels and chock are not required.
(3) You must ensure that trailers have working air
brakes, or another approved type. Air must be cut into the
trailer brake system at the time that the trailer is coupled
to the truck.
(4) The driver must test truck and trailer brakes before
driving down a steep grade.