Nissan Repair Questions? Ask a Mechanic for Answers ASAP
Hey there, thanks for the question.
as you probably know the system is sealed and should not be getting air back in the system unless there is an air leak. Without any fluid leaks, the only 2 things that are likely to cause this repeatedly are a leaking radiator cap with slightly low fluid or a small headgasket leak.
How many miles on the truck? and have you one a block test to heck for hydrocarbons in the coolant?