drztroy wrote:
the dealer i got my bike off told me its only ther incase you drop the bike ,, other wise there shouldnt be anything in it,
Your dealer is retarded :) You can turn the bike upside down and nothing will come out of the radiator.
The DRZ (like nearly every other bike / car made) has a pressurized cooling system. When it gets hot it pressurizes up to the relieving pressure of the cap. Usually 1.1bar (about 10kPa above atmospheric pressure). Once it goes above that pressure it pushes past the cap into the overflow tank. Then once it cools down it will try to go into negative pressure (slight vacuum). When this happens another flap valve in the radiator cap opens and lets the radiator suck the water back up, from the overflow bottle. If you don't have any fluid in the overflow, then it might be sucking air back into the radiator every time it cools down.
On a car, the cooling system is very large and there is a lot of expansion of the water. This means you will always see the overflow level rise as the car comes up to operating temperature, and go back down when it cools.
The DRZ has a tiny cooling system. It's level doesn't change much at all between cold and normal running. Expansion of the hoses and stuff almost compensate for the expansion of the water during normal running. So you don't need much in the overflow to ensure it won't suck up air when cold. But you will also have to get it very hot for it to boil and overflow an empty overflow bottle.