GMC TopKick 2007 Owner's Manual page 291

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.
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.
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.
2. Then keep turning the pressure cap slowly,
and remove it.
3. Fill the coolant surge tank with the proper
mixture, to the FULL COLD mark.
291