Wiz,
I respectfully suggest that the reservoir in not a pressure vessel and that a two position cover, while maybe a design by HD to make sure they don't open, makes no sense. Those covers don't seal and are only needed to prevent the coolant from sloshing out. The problem is that the reservoir is seeing pressure when the radiator cap fails during heat soak,sending coolant to the reservoir. No place to go but out the overflow and on the the ground. No one is saying their reservoir cap was off when this happened. Now, is this because there is still air in the system, is the cap radiator cap pressure rating to low, failing, defective, defective radiator neck or what, anybody's guess at this point.