Radiator expansion tanks

[Milky]

Hey all

Just a quick question on expansion tanks for radiators - If I understand their purpose correctly, they are supposed to catch all fluid that comes out of the radiator as it heats up, and then release it back into the system once things cool down again. Hence the requirement for level lines on the expansion tank that you keep the fluid between. The fluid coming out of the radiator makes sense to me, as the outlet into the expansion tank is above the seal on the radiator cap, so once pressure exceeds 12psi or so that the cap is rated to, it opens and allows the fluid to flow into the tank. On the way back the centre of the cap should be pulled down by vacuum and then suck the fluid out of the expansion tank.

My expansion tank was very full, and didnt reduce in volume when the bike cooled down. Hence I think that the return isnt working, which would be evidence of air leaks in the upper seal on the cap, or the return line right? Or am I missing something here?

[Motu]

Yeah,right I think - if the small valve in the centre of the cap is faulty it won't allow coolant back into the radiator,and a common symtom is squashed hoses as there is a vacuum in the system.Natural laws always follow the easiest route - so if there is a leak in the system then it will draw air from there instead of the more difficult task of dragging coolant back from the tank.So you can have a vacuum air leak that doesn't show up as a coolant leak even under pressure,it's easier to pass air than water through the gap.