GMC T-Series 2007 Owner's Manual page 218

Checking Coolant
CAUTION:
{
Turning the surge tank pressure cap when
the engine and radiator are hot can allow
steam and scalding liquids to blow out
and burn you badly. Never turn the surge
tank pressure cap — even a little — when
the engine and radiator are hot.
218
The coolant surge tank
pressure cap can be
accessed without tilting
the cab. It is located
behind the cab on
the driver's side of
the vehicle.
The vehicle must be on a level surface. When
your engine is cold, the coolant level should be at
the COLD FILL mark or higher.
See Low Coolant Warning Light on page 129 for
further information.
If the low coolant
light comes on in the
instrument panel cluster
and stays on, it means
you are low on engine
coolant.