GMC 1994 Yukon Owner's Manual page 226

Turn the pressure cap slowly to the left until it first stops. (Don't press
down while turning the pressure cap.)
If you hear a hiss, wait for that to stop. A hiss means there is still some
pressure left.
2.
Then keep
turning the cap,
but now push
down as you
turn
it. Remove the
pressure cap.
3. Open the air
bleed valve
located on the
thermostat
housing.
4. Fill the coolant
surge tank with
the proper mix,
up to the
COLD
mark.
I