yes it does, but not to the extent that it (the atmosphere) is repsonsible for keeping it closed.

The cap screws (cam action if you will) on an catches a lip on the radiator.

As you press down the cap and turn (on for instance) you are preloading the spring which forces the underside of the cap to catch the lip.

SO the expanding fluid of the radiator pushes against the spring which in turn pushes against the cap which in turn pulls against the lip of the radiator.

so the atmosphere has nothing to do with keeping the fluid in the radiator.

liquids are non compressable (relitivly speaking)

fill a 2 leater bottle (compleatly full) of water at sea level. take it to the mountain 8K ft, 10K or even space. As the pressure of the air surrounding the bottle holding the water is incapable of changing the VOLUME of water, then the pressure IN the bottle is always the same (even in space).

 

radiator blockage, 94 was boiling over, started with gurgling in the overflow. i smelled anti freeze, never seen it on ground. radiator was shot. replaced rad, works just fine

 

i agree the cap is responsible for keeping 15 psi in the radiator. but when the pressure inside the radiator and the atmosphere are in equilibrium, there are about 14.7 pounds per square inch of pressure inside the radiator already. at higher altitudes there will be less as atmosperic pressure is less. then, the cap is put on, adding 15 psi more "capacity" to the radiator. it all has to do with what the ambient pressure is when you put the cap on. so, if you put the cap on the radiator at sea level and brought it up to space and it only had a 12 pound rated cap on it, you dont think it would let pressure out? it would, until there was 12 psi in the radiator, when the spring in the cap would overcome the pressure in the radiator and keep it sealed. the atmospere and the cap work together to keep the atmospheric pressure plus the pressure exerted by the cap in.

say we change that 2 liter bottle (because it is somewhat rigid) and use two other examples.
1. a balloon: A balloon deforms pretty easily without a lot of pressure. 2 liters of water filling a balloon at sea level taken to space would turn to water vapor and expand the balloon until equilibrium is reached. (the pressure the balloon exerts on the water vapor would equal the pressure the water vapor exerts on the balloon)
2. a completely rigid container: If the container were filled with 2 liters of water at sea level and taken to space, there would still be 14.7 psi of pressure inside the container, as the container is able to exert 14.7 psi on the contents even when the pressure outside the container is almost zero.

the ability to pressurize fluid without a big change in volume is why hydraulic machines work. there is still pressure, but not volume change

the reason water changes to water vapor under no pressure is shown by the phase diagram for water