GMC C-Series 2001 Owner's Manual page 110

Apply Park Brake Warning Light (Option)
When it comes on, you will also hear a warning tone.
Leaving the vehicle in this condition could result in your
vehicle moving and will discharge the battery.
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This light warns the driver
that the ignition has been
turned off without applying
the parking brake. The
warning light should come
on, as a check, when you
start the engine.
Parking Brake Warning Light
When the SERVICE PARK BRAKE light comes on,
you will also hear a warning tone. If this light comes on,
have the system repaired immediately.
This light turns on as a
check when the engine is
first started and any time the
system needs adjustment or
there is a malfunction in the
electric/hydraulic parking
brake system.