Global Machinery Company Sierra 1999 - Manual (Page 277)

yellow blue
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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 the FILL COLD mark.
5. Then replace the pressure cap. Be sure the pressure
cap is hand - tight.