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You've described some a-typical blown head gasket symptoms.
Low coolant without any obvious leaks, high pressures (sometimes damaging heater cores or the rad), 'milky' or 'brown' substance in either the overflow or in the rad itself.
You can check under the oil cap, or on the dip stick for 'milky' looking oil or foam, though most people have to drain it to see. Pulling the plugs and checking for white deposits might be another route to check, or taking the truck to a mechanic with a combustion gas detection kit for the coolant system.
Paul
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