It is a little different than that even.
The train has one pipe, the whole length. No separate supply/apply lines.
If you increase air pressure to the cars, by any amount, they all release completely dumping their own air reservoirs into the line to help it release faster. You then continue to supply air to the line/reservoirs and pump them all back up to full pressure, say 80 psi, whatever the company policy is.
Now when you want to make an application you release air from the train line and the magic valves give you 2.5x the reduction amount in the brake cylinders. So you drop air 10 psi so the train line and get 25 psi to the brakes.
But the one and only way to set air brakes on the cars is via a reduction on the line. You can't continue to supply air to them for parking even if the loco is running since any supply of air would cause a full release. So once they leak away, they begin to roll free again. There is nothing like a johnny bar to the cars on the train.