GMC T-Series 2007 Owner's Manual page 224

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. Do not
spill coolant on a hot engine.
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
about one full turn.
If you hear a hiss, wait for that to stop. A hiss
means there is still some pressure left.
2. Then keep turning the pressure cap slowly,
and remove it.
224
3. Fill the coolant surge tank with the proper
mixture, to the COLD FULL mark.