Careful! The cooling system cannot be reliably topped off and certainly not filled via the plastic overflow tank. I know of engines killed by this point of confusion. Even a small amount of air connecting them will cause problems. New installations or a drain/refill of the system introduces many places for bubbles to hide and it takes a while to for them to end up in the expansion tank. When that happens, the fluid connection between there and the overflow is lost and problems ensue.
1) A hint that it's working properly is that the level in the overflow rises by a couple of centimeters as the engine warms up, and drops to the original level when cold.
2) A hint that the cap is going bad is when some coolant gets blown out the little vent hole when running. (Or could be from a cracked cylinder head).
3) Even tiny leaks can introduce air into the system. These are easy to get and difficult to find after a hose change.
4) With any part of the connecting hose below the nipple on the overflow tank, air gets into that hose. If all is working well, the expansion from the next time the engine is run will tend to push it out. But that's not guaranteed. To reduce or eliminate the chance of that happening, I get all OCD and do the final bit of expansion tank fill *from the overflow bottle*. Using a bicycle pump, basketball filling needle and chewing gum, I get enough of a seal on the little vent hole to slightly pressurize the bottle and force fluid up into the tank, then slap on the cap as it reaches maximally full or above.
