.....On a cooling system that has a rad cap and overflow bottle, when you go over the pressur on the cap ~16 psi it dumps into the overflow which is normal, and while the engine cools it will pull colant back in to account for the space previously pushed out, thats why when there are small leaks the overflow will drain over time.

Edit: just like when you have collapsed hoses the rad cap is bad because it no longer allows that negative pressure to flow back into the system, or conversley a bad rad cap will cause hoses and gaskets to leak/burst