The radiator cap on my rig is spring loaded so that when the pressure is high it allows coolant to squirt out into the plastic reservoir. The theory is that when the engine cools down it will suck the excess collant back into the radiator. What I don’t understand is how it sucks it back in. The gasket, under suction, will seal even better. Anybody able to explain how the coolant gets back in to the radiator??? thanks!!
The same way you can suck up juice with a straw. When the coolant in the radiator cools off, so does the air above it and as a result, its pressure drops. The lower pressure inside the radiator allows the pressure of the atmosphere outside to push the fluid back out of the reservoir and into the radiator through the overflow tube. This, of course, assumes that nothing leaks.
Wait, I don’t think that’s right. The pressure in the radiator can’t get very much below atmospheric pressure anyway, can it? Gonna have to retract my answer above, for now. I might be making some bad assumptions here.
So it does work the way I stated above. 
Good. For a second there, I thought you were wrong, and the world around me was crashing down. Thank god for normalicy.
If you look at how a radiator cap is made, it is very simple. You have a round brass disc with a rubber gasket that is about the same size. Through these is a brass pin with a small brass disc attached. The upper end of the brass pin hooks into the top of the spring. As the radiator cools a vacuum is created and coolant is pulled through the center of the large brass disc and rubber gasket around the small disc. Most folks think the small brass disc is to hold the rubber gasket to the large brass disc, it also serves a dual purpose.
I knew that $50 I spent to attend a Stant training seminar would pay off some day!!!
The large rubber gasket seals against the radiator neck. It is held there by a stout spring that is overcome at the cap’s pressure rating, typically about 13-15 psi or 1 bar. The return flow does not go through the same portion of the cap. It goes through the smaller valve in the center, as racer72 indicated. Some of these small central valves have a light spring holding them against the underside of the cap, but it seems most are free-floating and rely on system pressure to hold them up to seal. In a nushell, under system pressure the coolant is forced around the edge of the cap, when the systems goes to vacuum the coolant returns through the center of the cap.