Parking Brakes and Emergency Stop Procedures
Parking and emergency-stop questions check whether you know which control does what and when it is safe to release it.
The yellow knob
The yellow diamond parking valve is pulled out to apply and pushed in to release. The spring brakes do the holding; air releases them. Push in only when pressure is up (typically above 100 psi on the gauge); releasing with low pressure leaves the brakes dragging or not fully released.
Parking on a grade
Set the parking brake, turn the wheels toward the curb (or into the shoulder if there is no curb), and chock if heavily loaded. On a tractor-trailer the trailer parking brakes set with the same knob on most units. Never use the trailer hand valve or the foot pedal alone to park.
Emergency stop
If service brakes fade or fail, use the parking valve progressively - a hard pull can lock the drives and jackknife. Keep both hands on the wheel, downshift, and steer for the shoulder. If air is lost, the spring brakes will apply whether you ask or not; your job is to control the path.
Worked example
After a long hard pull you park on a ramp and the truck creeps after you set the yellow knob. Reasoning: check that the knob is fully out and pressure has dropped enough for the springs to set; on a steep grade, chock the wheels as well, because spring brakes alone may not hold a heavy load. Do not rely on leaving it in gear.
Traps
Releasing the parking brake before pressure has built; using the foot pedal as a parking brake; yanking the yellow knob to the full-out position when controlled emergency braking was needed.