Okay, that sounds promising.
While moving cars about last night, i left the Truck idling, 30 minutes later went to jump in and switch off. I noticed the defrost was cold and the temp gauge reading a touch under the red. I cut the engine to hear that ominous sound of boiling coolant and it blowing out the overflow. I fired up the motor and kept the revs up around 2500 and immediately watched the temperature gauge drop to halfway and heat coming from the defrost vents. Leaving it running the coolant level was halfway down from the full mark in the header tank, hoses were nice and pliable to touch, not hot, sides of the radiator too not hot, center of the radiator was stone cold, both Heater pipes cold. The heater was blow cold inside, but applying revs, the heater will blow hot, back at idle it was cold again.
I'm guessing that the heater operation is working and now my thoughts are turning to the water pump is on early failure. not noisy or leaking, just not pumping enough coolant.

 

is how I repaired my blend door. I used a Oscillating multi-tool to make a cleaner cut to enlarge the opening.

 

Many thanks for this link, well done. We are going to try one more option before checking the door flap, as the one I check previously must have been the hot and cold heater control **** which seemed to work fine.
One other thing, If this recirculating door is broken would it still open and close by increasing the engine rpm's?
The heater vents blow hot air, after idle the heat goes and cold air blows out. Take the revs above 4000 and the heater blows hot again.
We are going to look at fitting a new water pump first as I don't believe there is enough flow, and I have noticed a howling noise coming from the front of the motor when cold up to normal temp.

 

My door broke and fell covering the blower motor and was always pulling in outside air.

 

Everything is as it should be, heater controls all opening and closing. Our problem is a blocked heater matrix. The main radiator was 80% blocked, so we took our thoughts to the matrix. Coolant isn't circulating unless hi revs are applied then there is some coolant coming through but not a descent flow.

 

So is it fixed?

 

No, not yet. It's on hold as I have other work that's taking priority.

 

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restricted heater core seems to be common, like the radiator, of fouling if the coolant is run too long between changing (change at 3-4 years). Disconnecting the heater hoses at the water pump and then blasting through those hoses with high volume snd pressure from a garden or shop hose will usually knock alot of gunk out of it and get it working properly in short order.

as for bleed air out. Happens automatic. The thermostat should have a jiggle pin in the flange. When tstat is closed That jiggle pin is supposed to let the air out of the circuit and sends that air to the top of the rad. I do not trust the jiggle pins and I always cut them off and leave that little hole open full-time.
The air burps up and will come out either open rad cap or pushes out through the reservoir if enough pressure builds. When there is alot of air, the pressure does not get high enough for that. So the first step is to run at idle with rad cap off and keep topping up through the open cap hole as the air burbles out. When it is warm enough that the tstat opens the last of the air comes. At that point top the rad through the cap hole, put the cap on, then fill the reservoir to overfull. Next go for a 15 minute drive, watch the gauge temp to not overheat from an air pocket. Vary rpm on the drive. Remaining air comes out through the reservoir on your drive. When you park and engine cools, the pressure will drop and it will suck in coolant from reservoir. Top off the reservoir after parked and cooled. Keep a jug of coolant in the truck for a few drive cycles and top off the reservoir at each drive before starting the truck. Eventually all the air is out and reserve level doesnt change.

all of that is assuming there are no external leaks in the coolant circuit or an engine problem such as head gaskets. A head gasket leak will push exhaust gases INTO the coolant system, over pressuring it, causing gas/air pockets, and pushing coolant out of the reservoir. A blown head gasket will behave like always bunch of air in system and constantly low on coolant. The exhaust may also be blowing some white smoke and smell sweet.

Based on the dates at this time you must have this solved by now. If not, or just for a general reference, hope that helps!