kaydeejay,
Two very different scenarios!
Bricks in a truck's bed are only weight on the truck's rear axle and tires and their entire weight is stopped soley by the truck's brakes and by nothing else.
The 5th wheel pin equal weight on a truck's rear is attached to and part of the 5th wheel and is soley controlled by the brakes on the 5th wheel only. There is NO seperate forward push from the pin weight as with the bricks. The pin weight is controlled and stopped by it's very own trailer's brakes. If anything, the weight of the pin gives the truck's rear tires the same additional traction as the bricks do but yet adds zero forward inertia requiring more stopping ability from the truck's brakes. Actually less truck stopping power needed as the 5th wheel brakes pull backward on the truck and the hitch pin when engaged. Think about it as it's factual!