Allows the spring brakes to be manually released in an emergency. And an emergency cannot be stressed enough. Unless you are in the middle of Plumb 40 (Plumb in the middle of no where and 40 miles from everywhere), and in peril, do not cage the spring brakes.
Simply speaking, you are under the truck with a wrench attempting to cage the brakes and something happens – the brake chamber explodes and removes your head from your shoulders.
Miguel the Mechanic comes out with his impact wrench and has to cage the brake. Miguel uses the impact at arm’s length and if the brake chamber explodes, he’s only going to lose his hand as opposed to his head.
It is simply not worth the risk, especially in this day and age of North American-wide mobile mechanic networks.
Again – don’t manually cage a brake unless it is a dire emergency!!