Hmm. Some air brakes uses spring actuation. When you let off the
air inside the system, theres nothing to hold the spring back,
hence the spring forms as a brake as the springs pushes the brake
pads against the drum brake/disc brakes. Whether you release air
for parking brake or if theres a leak in the system, the springs
will always act as a secondary safety device.
Yes, you are right. That was what I was trying to say. I didn't
mention how exactly the high air pressure transmits its energy to
release the brakes in my earlier post.