Parking Brakes, Retarders, and Anti-Lock Brake Systems
Parking and spring brakes
The parking brake on an air-brake vehicle is the spring brake: a heavy spring applies it and air pressure releases it. Pushing the dash knob in charges the chambers and releases the brake; pulling it out exhausts air and applies the brake. Because it is spring-applied, it sets itself if air is lost - that is why it doubles as the emergency brake. Always chock wheels and set the parking brake before leaving the cab, and remember that on some vehicles the parking brake also applies the trailer brakes through the same control.
Retarders
A retarder (engine brake, exhaust brake, transmission retarder, or driveline retarder) slows the vehicle without using the service brakes, saving air and lining life on long grades. It is not a parking brake and not a substitute for service brakes in a panic stop. On slippery surfaces, some retarders can cause drive-wheel skids; many automatically disengage when the ABS activates or when the wheels slip. Turn it off on ice and snow unless the manufacturer says otherwise.
Anti-lock brake systems
ABS keeps wheels from locking during hard braking so you can steer. It uses wheel speed sensors and a controller to modulate pressure. When ABS works, you may feel the pedal pulse and hear a chuffing sound - that is normal; keep firm, steady pressure and do not pump. An ABS warning light means the system may not function, but you still have normal brakes; get it checked. ABS does not shorten stopping distance on all surfaces and does not let you drive faster.
Traps and a rule of thumb
Questions confuse the retarder with the parking brake, or claim ABS means you can steer and stop on ice at any speed. Another trap: pumping the pedal during ABS activation defeats the system.
Rule of thumb: spring brake parks and rescues; retarder saves brakes on grades; ABS keeps you steering - press and hold.