Chevrolet Express Owner's Manual page 275

1. Remove the radiator pressure
cap when the cooling system,
including the radiator pressure
cap and upper radiator hose, is
no longer hot. Turn the pressure
cap slowly counterclockwise
until it first stops. Do not press
down while turning the
pressure cap.
If a hiss is heard, wait for that to
stop. A hiss means there is still
some pressure left.
2. Keep turning the pressure cap,
but now push down as you turn
it. Remove the pressure cap.
3. Fill the radiator with the proper
DEX-COOL coolant mixture, up
to the base of the filler neck.
See
more information about the
proper coolant mixture.
4. Fill the coolant recovery tank to
the COLD FILL mark.
5. Reinstall the cap back on the
coolant recovery tank, but leave
for
the radiator pressure cap off.