[Coley]

Is the truck and its hose and cooling system fully original?
in other words are all the hoses connected to the right heater and bypass points?....or have any of the hoses being re-routed due to a heater core failure or whatever else reason?
Have you checked the thermostat to make sure it is functioning correctly?...I think it would normally be found inside the t-stat housing. Its easy to check....remove it and put it in a pot of boiling water....it should open up.
Water is not compressible, so if it can't get through somewhere....then the pressure that will build up behind increases...a lot and it will blow through something.

my two bits...so far

Coley