The RADIATOR CAP should be providing he pressure and pressure relief.
With a 0.9 PSI cap you should be able to squeeze the upper radiator hose together with your hand. With 16 PSI cap it should be very firm.
After the car cools to basically room temperature leave the overflow cap off and let it reach operating temp (idling should do it) then feel the upper hose again. If you can squeeze it easily then the overflow does not have a vent. You can check the overflow tank as I described earlier by disconnecting the small hose under the rad cap and putting air pressure to the small hose. With the overflow cap off you can blow into the hose and hear bubbles. With the overflow cap on and tight you can not blow air into the hose it is sealed or you are not blowing hard enough and you need to test it up to 20lbs.
This is an extreme example. in our cooling systems a rupture will happen before an explosion. BUT I have been burnt by 200 deg coolant and its not fun.