GMC C series 2002 Owner's Manual page 87

Trailer Brake Hand Control
Valve (Option)
If your vehicle has this feature, the control is mounted
on the steering column.
2-44
This feature lets you apply the trailer brakes without
applying the tractor brakes. You can apply the trailer
brakes a little or apply them all the way if you have to.
Use this control only when you're driving. Don't use it
for parking or to hold the rig on a hill.
CAUTION:
Using the trailer brake hand control for parking
or for holding the vehicle on a hill may not keep
the vehicle from rolling. This can happen if
someone hits the valve by accident, or if air
pressure bleeds from the system. If the vehicle
rolls, you or others could be injured. To park
the vehicle or hold it on a hill, use the parking
brake properly.