Engine/Transmission (1998.5 - 2002) coolant level problem

[Texas Diesel]

The coolant level in the *radiator* is down 3/4" everymorning. I top the radiator off and the next time I check it it is low. The overflow bottle keeps rising because I am adding fluid to the radiator that is getting pushed out and not sucked back in when it cools. I've tried 3 different caps thinking they were letting air get sucked back in. My wifes radiator is full every time I check it so it is something with mine.