1999 Cadillac DeVille Owner's Manual - Page 254
1999 Cadillac DeVille Manual
Page 254 highlights
yellowblue 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) 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. 5-16