GMC 2003 GMT560 T-Series Owner's Manual page 182

3. Then fill the coolant surge tank with the proper
mixture, to the FULL mark.
5-26
4. With the coolant surge tank pressure cap off, start
the engine and let it run until you can feel the upper
radiator hose getting hot. Watch out for the
engine cooling fan.
By this time, the coolant level inside the coolant
surge tank may be lower. If the level is lower, add
more of the proper mixture to the coolant surge tank
until the level reaches slightly above the FULL mark.