Ever wondered why the overflow tank is so large yet the full line is so low?? It's the same reason why you don't use a hose on the cap. That amount of air space is required for expansion & contraction of the coolant. If you put a hose on to the cap you WILL run your coolant level down over a period of time & ultimately cause overheating problems. Like Shadow, don't ask me how I know

 

Same thing happens if you overfill the tank - no room for expansion = green goo all over the bike..................