Feel both heater hoses, they should be close to the same temperature. The larger 3/4%26quot; hose is the inlet and should feel slightly warmer than the smaller 5/8' outlet. On most full size pick up trucks, the positioning of the heater core places it higher then the radiator fill and can trap air in the heater core, this will usually give some heat for a moment then go cold. This doesn't sound like what you are explaining.
It sounds like there is some blockage in the heater core. Flushing the heater core actually is a good idea, but make sure to back flush it too.
When purging the coolant system, make sure the heater, not defrost, is on and the fan is on low. this will open the heater control valve and allow air to be forced out of the heater core.
The only other thing I can think of that could be a problem, is a bad or weak radiator cap.
Hope this helps. ChristopherNo heat in my 97 ford f150... changed thermostat..?probably have a blend door broken, or blend door motor frozen, if you're lucky its the heater control switchNo heat in my 97 ford f150... changed thermostat..?Question have you ever flushed out the cooling system and put in anti freeze yet?The problem you might have is that the radiator might be plugged up, you think of changing the radiator for a new one.Because if you haven't flushed it out in awhile like years it might be that the reason for a plugged up radiator is the radiator get stuff built up on the inside of it and can't pass the coolant very well through the inside fins of the radiator and secondly when you refilled the radiator did you leave the engine run while filling it up because you might have air trap inside the cooling system and this can cause it not to heat up either but leave the heater on high while filling it up with anti freeze this will push the coolant through the system and get the air out and hope to get the heat you need.These are good type of engine that don't get the coolant through them very well air get trap somewhere in the system and can't get out while refilling them with anti freeze or coolant.Try these methods and see what happens.Good luck! firstname.lastname@example.org