Global Machinery Company Sierra 1999 - Manual (Page 276)

yellow blue
5-16
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 (left) 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.
3. Then fill the coolant surge tank with the proper
mixture, to the FILL COLD mark.