The brakes will always squeal. This causes friction which slows the vehicle and creates heat. This causes heat which automatically causes the brakes to lock up. Air pressure forces out the push rod and moves the slack adjuster. Air pressure works only with wedge brakes and not with S-Cam brakes.
Air pressure is converted into hydraulic pressure.
It presses them against the outside of the brake drum. It presses them against the inside of the brake drum. It presses them around the brake drum. The slack adjusters. The alcohol evaporator.