I have replaced the heater core on my 1992 F150 5 times since the original one started leaking 4 years ago. The last one only lasted 2 months. Any ideas? I am wondering if maybe I reversed the 2 hoses that connect in the engine compartment. I hope its not something more serious...like a head gasket leaking air pressure into the cooling system. No water shows up in the motor oil though..Very puzzling.

 

Have you checked the release pressure of the radiator cap? Seems the heater cores are a weak point.
Good luck

 

Thanks J. Low. I will replace the cap...