Hopefully the truck has BFS and there will not be any interior rust in the lines and it will be surface rust that needs to be looked over. Somehow the moisture that does migrate in seems to get to the wheel cylinders and the master cylinder bores more than anywhere else.
My truck has been thoroughly bled and flushed and Since I did the complete rework of the system have been good.
At some point when you get caught up, there are some PMCS tasks that you can do that require you to pull the wheels and hubs so you can grease the bearings and inspect the brakes, shoes, wheel cylinders and drums.
Don't let me get to far ahead of you....let's get this problem fixed first!