Buick 1994 LeSabre Owner's Manual page 200

How to Add Coolant to
the Radiator
1. You can 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 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.
3.
Then keep turning
the pressure cap,
but now push down
as you turn it.
Remove the
pressure cap.
Fill the radiator
with the proper
mix, up to the base
of the filler neck.