I beleive it is a spring loaded pin that holds the outside of the drum. The spring is there so if it is put in park while moving, it will bounce out until the truck stops....
I have seen them fail and break, BUT that is normally Park at 60MPH or simply worn out from abuse or severe over loading in park...
My driveway is steep also..... there are times I don't set the parking brake, But I like to set the brake first to hold the weight of the truck and then put it in park...