Twenty five years ago, I worked for a dairy, delivering milk, door to door, when the regular drivers were off. The trucks were curbside delivery vehicles with sliding doors on the cabs. Some of them had 'Mico Lock" park brakes, a valve on the brake line to hold pressure after you took your foot off of the pedal.
Rolling to a stop in front of a house one afternoon, I set the brake lock and picked up a carton of milk. This customer used to put tokens in a small jar on a stump in the front yard. As I approached the stump. I realized there was no token, so turned around in time to see my truck rolling backward down the street! I realized I had only put enough pressure on the brakes to halt the truck's forward motion as it rolled to a stop, but NOT enough to keep it from rolling back down the hill!
Dropping the carton, I sprinted out the driveway and across the road, aiming for the open side door. I managed to dive into the truck, slam both hands down on the brake pedal and bring the thing to a crashing halt, still in its lane and on course for the intersection and embankment about 100 yards further back.
All I had to do after that was pick up the tumbled milk cases in the back, retrieve the carton from the lawn and nurse my sore ribs for a week or so.
Another driver told me he had been in the back of his truck in a similar situation. Needless to say, these vehicles no longer rely upon brake line locks.
The main difference between genius and stupidity is: Genius has limits.
Les