I assumed the brakes were applied unless air pressure was available. Same as truck / trailer brakes. In the article it references air pressure is required for the brakes to be applied - is there a reason it can't be the other way around?
It'd be great if you could use a spring to apply the brakes and an air cylinder to hold the brakes open. But the force needed to apply brakes is too much for springs, so it uses an air cylinder to apply the force. There's a complicated system where under normal conditions, pressure in the hose running the length of the train is fed into a reservoir in each car. When the pressure in the line drops (either a disconnect or the engineer hitting the brakes), a valve connects the brake cylinders to the reservoir and the air pressure applies the brakes.
So yes, if you leave some isolated rail cars sitting there with the air brakes applied, the pressure will slowly leak away over hours or days and the car will start rolling.
However, this was about the ONLY major advantage it had, and decades of pro-westinghouse propaganda against it blew the significance of all of the disadvantages* up to be greater than the advantage of fail-safe, and we switched to the westinghouse system.
* One of the biggest claimed disadvantages is the fact that vacuum brakes are less effective as you increase altitude, this is a HUGE disadvantage to operating vacuum brakes in the US where a large amount of rail traffic is at altitude, but in the UK where our highest peaks are 3000' and we don't generally run railways to the peaks of those hills, it's almost a non-issue.
Passively-applied brakes would break a ton of standard railroad operations which depend on cars rolling free, such as in "hump" classification yards which use in-track retarders to control speed, and in other situations such as "kicking" cars into tied-down cuts.
"Though exhausted after the journey, Mr. Harding took the laborious step of setting the mechanical hand brakes — a train’s version of an automobile parking brake — on the five lead locomotives, an equipment car and an empty boxcar.
That was the first big mistake. Investigators calculated that Mr. Harding should have also secured the hand brakes on 18 to 26 of the tank cars before retiring to his hotel."