Applies and releases the truck (tractor spring brakes.
It is a yellow 4-sided button on the dashboard.
If your vehicle has a air brake equipped trailer attached, the tractor parking brake will also cause the trailer supply valve to apply when the truck parking brake is applied. The trailer air supply valve has to be held in if the driver wants to apply the truck parking brakes independent of the trailer ie: in the case of completeing tug tests for the purpose of a road test.
If you forget how the button works you can always read what's written on the button: "Pull to Apply; Push to Release." Although somewhat cynical, reading doesn't always work because the text wears off over time.
Another way to remember is:
The parking brake lever in a passenger car:
- Up or out to apply
- Down or in to relase
You pull it up to apply the parking brake and push it down to release the parking brake - the truck parking brake works in a similar manner.
When the driver pushes the button in, it fills the truck's rear spring brake chambers with air and thus cages the parking springs. Consequently the parking brakes are released. When the button is pulled out, the air in the spring brake chambers is released via the quick release valve and the spring expands and applies the parking brakes.