First make sure the coolant is full-- and you have bleed the system of air. That is the small bolt at the front of the thermostats. Do you have to add coolant regularly-?
If you have air, you might have to drive it and bleed again.
After you drive the truck, and is parked overnight-- before you start it--check the upper radiator hose. Is it easy the compress-? That may indicate you have a bad head gasket. Which I suspect is your problem.
Is it collapsed-? That is caused by a sticky reservoir cap-- wash with soapy water or replace.
Chevy 04.5 LLY, 2X, shell, XDR, MikeL Heavy duty up-grade, 6spd, TransGo, deep pan, de-badged, 65 Gallon replacement Aero tank, LBZ-mp, Kennedy pump, shimmed, Pre-fuel filter, tow mirrors, BU cam, gauges, GPS, Sirius, bags, custom grille, fender vents, HD trans cooler- Curt front hitch, footwell lights, load lights, Studs, head gaskets.--- Jerry