How does the the overflow bottle work in our bikes cooling system? Like all physical matter, liquid expands with heat. As the coolant in the radiator and the bike cooling system starts to heat up and/or boil, it will want to pressurize the cooling system and find a way to escape. To avoid blowing a radiator hose off you engine, the radiator cap has been designed with a two-way vent system. The spring-loaded rubber seal inside the top of the radiator achieves the first stage of the cap's system to run at about 15psi, at which point it overcomes the spring tension and lets the pressure escape before it blows a radiator hose.
This escaping fluid/pressure is directed straight into the overflow tube, which vents between the inner and outer seal of the radiator cap.
The reservoir bottle at the other end acts as the cooling-off chamber where all the steam can precipitate and return to its liquid form.
Ever stopped at the top of a monumental hill climb and heard your bike bubbling away like Bongin' Billy in the back shed? That's your overflow bottle doing its job. At this point, those bikes without the overflow bottle would be pissing a plume of coolant steam out through a pipe venting at the bottom of their engine.
It doesn't end there. As the engine begins to cool, the expanded coolant begins to contract, which creates a vacuum in the cooling system. In turn, the cooling system sucks on the overflow bottle to refill its own radiators. It's able to do this as the base of the radiator cap has a one-way valve inside it. The little brass dish that holds the rubber seal in place is actually the coolant return valve.