Can someone explain how the radiator cap and over flow system are supposed to work? I just got a new radiator. With the radiator sitting on the garage floor I installed the cap. I thought if I blow into the overflow tube it should be blocked. So, I tried it, but the tube is open to the radiator and I can blow straight through. Does this mean I have the wrong cap? I noticed the radiator cap has 2 different sealing surfaces. One is above the overflow hole and the other is below it. When do the seals open to vent pressure or fluid?