Durango overheating question.

Sooooo. First thing. I know for sure it never boiled over today. Its cold now and just removed the rad cap. Not running in laneway. Serious sucking sound when removing cap. Down 4 liters (1 gallon) of antifreeze! Overflow still full. Refiled rad and now interior heat and gauge running normal. Thinking bad cap not returning coolant from overflow.

Just drove 1 hour all good. Ordered new cap. Hopefully an $8 fix!