Compare hydraulic brakes to air brakes?

In a vehicle with hydraulic brakes when you step on the brake peddle it pushes fluid thru the brake lines which forces the brake shoes onto the drum or caliper to apply the brakes. With air brakes like a semi when the system is depressurized ( no air in system) the pads are pushed tight against the drums. When the driver releases the parking brakes they are putting air into the system counteracting the force of the springs holding the shoes to the drums and pulling the shoes off the drums. (bear with me) When the driver pushes the brake peddle it releases some of that air allowing the shoes to contact the drums again applying the brakes. This is so if you lose all air pressure the brakes lock up. Older trucks (pre 70's) were built opposite so if you lost air you had no brakes