Overheating

How do you know it is building up too much pressure? What is the condition of the coolant? Is it rusty or have oil in it? Is it circulating through the engine? Is the heater working when placed on HOT and fan on?

It would run for two or three hours and then the cap would release pressure and then it over heats. The coolant is new, the fan runs, thare no bubbles in the line and the heater works fine

If the fan is operated via a belt, make sure the fan clutch is not shot preventing proper circulation of air. If electric and has two fans make sure both are working. The three hour portion confuses me. If a serious cooling component failure is occurring it should not take two to three hours to overheat.

The fan is clutched and its runs all the time and I made shur that thare no blockages in the air flow. I ran it on a reader and it siad that the overheat protection was active

Are you getting a light on the dash indicating overheat? Was the code something like 1299, cylinder head temp sensor indicating an overheat condition, Is your truck equipped with "limp mode"? If so, is it going into limp mode? If you are positive it is overheating and not getting an idiot light or temp gauge increasing, try changing the CHT sensor and check for electrical issues at the connection point of connection, making sure ground is good. As I stated before the two to three hour constant driving fact is an issue. If there was an bad issue in the cooling system it would not take two to three hours to show itself even if left at idle. Since the thermostat is a spring operated mechanical device, you can try replacing it, they are not expensive. Also note engines, even as old as yours, will overheat quickly if water is just added to the system. If you are not running a 50/50 mix you could have that problem.