Radiator overflow not returning fluids

[landarts]

I have a 72 1/2ton Cheyenne that I am in the process of restoring. It had the original radiator overflow tank in the truck when I bought years ago. When I drive for a bit it dumps fluid from the radiator into the tank. I guess thats normal from what I have read on the forum. However that fluid never returns back to the radiator at any given time. So usually after a few days of driving I siphon that fluid into a used anti-freeze bottle and dump it back into the radiator. Do you think there could be something wrong with the cap on the overflow tank causing it not to return. Or am I missing something else ?