Hydraulic and air brake systems compared

(a) Hydraulic brake system

It works on Pascal's law: pressure applied to a confined fluid is transmitted equally in all directions.

 Pedal -> [Master cylinder] ==brake fluid in pipes==> 
                       |-> wheel cylinder (drum) x2 (rear)
                       |-> caliper piston (disc) x2 (front)

1. When the driver presses the pedal, the push rod moves the piston in the master cylinder, which builds up fluid pressure (about 40 to 100 bar in hard braking).
2. The pressure is carried through steel pipes and flexible hoses to the wheel cylinders (drum brakes) or caliper pistons (disc brakes).
3. The pistons push the brake shoes against the drum or the pads against the disc; friction slows the wheel.
4. When the pedal is released, the return springs (and the piston seals in disc calipers) pull the shoes and pistons back, and the fluid returns to the master cylinder.

A vacuum booster multiplies the pedal force. A tandem master cylinder gives two separate circuits (front/rear or diagonal), so that braking still works if one circuit fails. Brake fluid is DOT 3/4, glycol-based, with a high boiling point.

Force multiplication: $F_{out} = F_{in} \times \dfrac{A_{wheel}}{A_{master}}$.

(b) Pneumatic (air) brake

 Compressor -> reservoir (8 bar) -> brake valve (pedal)
        -> air lines -> brake chambers -> push rod/slack adjuster
        -> S-cam -> brake shoes on drum

• An engine-driven compressor fills the air tanks to about 8 bar.
• The pedal opens a brake valve that lets air into the brake chambers; the diaphragm pushes a rod that turns the cam (or wedge) and applies the shoes.
• When the pedal is released, the air is exhausted and springs return the shoes.
• On trailers and long vehicles, a relay valve applies the brakes quickly. Spring brakes apply automatically if air pressure is lost, which is a safety feature.

Reasons for use on heavy vehicles: an unlimited supply of air, high braking force through large chambers, no risk from fluid leakage, easy coupling of trailers, and light driver effort. The drawbacks are bulk, cost and a slight delay in application.