GMC C-Series 2001 Owner's Manual page 197

CAUTION:
You can be burned if you spill coolant on hot
engine parts. Coolant contains ethylene glycol
and it will burn if the engine parts are hot
enough. Don't spill coolant on a hot engine.
5-30
1. You can remove the coolant surge tank pressure cap
when the cooling system, including the coolant surge
tank pressure cap and upper radiator hose, is no
longer hot. Turn the pressure cap slowly
counterclockwise until it first stops. (Don't press
down while turning the pressure cap.)
If you hear a hiss, wait for that to stop. A hiss means
there is still some pressure left.