loosing anteefreeze

Does it build pressure in the radiator after it warms up? I've had one only loose antifreeze after it warmed up and put compression into the cooling system trikling out the overflow.

 

Put a catch jug on the overflow hose and monitor it for collecting coolant. Consider a new radiator cap. Check the air tanks for coolant.

 

It might be leaking into a combustion chamber/cylinder and burning off.
That's the only other place it would go.
A shop should be able to detect water/anti-freeze burning through analyzing your exhaust gases.
Also, check your oil for signs of water/anti-freeze.

 

Do you see any unusual smoke from the pipes? Do you have a trans cooler? Have you considered the heater core/cores?

 

You might pickup a hint of hot antifreeze odor in the exhaust flow if it is going out that way, and like bender said about the heater cores, the bunk heater is far enough back that you might not notice a small puddle under the truck. I have followed trucks before that I could smell hot antifreeze in the exaust, you might get someone following you to check also.

 

We had a truck in the shop recently that the owner kept adding antifreeze to. Pressure test found nothing. I happened to see a drop of water on radiator. Added a bottle of K-Seal and no more issues.

I would try the catch jug as well as a new radiator cap as bender suggested. Then maybe try the K-Seal.

 

That happens a lot more than you would think, radiator slightly leaking and it doesn't show up until you start seeing the green spot around the leak, if it is in an area that is visible. When the radiator gets hot the material expands and leaks more, then you pressure check it when you get in and usually the system is not up to running temperature, so it doesn't show up as well.