What is the purpose of spring brakes on a commercial vehicle?

Spring brakes serve two functions: they are the parking brake (engaged when the driver pulls the yellow control), and they are the emergency brake that automatically applies if the air pressure in the brake system drops below a safe level (typically 20–45 psi). Spring brakes are powered by mechanical springs — when air pressure holds the springs back, the brakes are released; when air pressure fails, the springs apply the brakes. This 'fail-safe' design ensures a runaway truck does not run away even if the entire air system fails. You should not press the brake pedal while spring brakes are applied, because doing so can damage the brake mechanism by pushing the chamber against an already applied brake. The spring brakes are completely separate from the air supply that powers the service brakes.