take it off and inspect it - it has two hats one larger with ears that you grab to open and one further down, both have a spring in between, the spring lifts and lets the coolant/water escape when it expands under heat - this is usually 13.9 psi. This is called the pressure relief valve.
There is another valve in the shape of a hat - it opens downwards and may or may not have a spring. This functions when the water/coolant shrinks when cooled down and a vacuum is created in the system. The expanded coolant is then sucked back into the system to top it up.
If your expansion pipe has a pin size puncture it will not suck the water back and will suck air from that hole (imagine it like a straw while sipping your coke) - if your straw has a hole you wont get coke, if your straw is not dipped into the liquid you wont get coke, same principle.
