Are Semi Trailer Brakes operated with Air?

Posted by at 16 May 2014, at 18 : 02 PM

Are Semi Trailer Brakes operated with Air?

The words Air Brakes and Semi Trailer Brakes are almost always interchanged when we speak about over the road commercial trucks. Air brakes are the most powerful type of trailer brakes and anyone operating a vehicle with air brakes must be certified to do so. Any vehicle large enough to require air brakes will require that the operator of that vehicle must hold a commercial drivers license or CDL as it’s called for short.

Air Semi Trailer Brakes use the force of air pressure to activate the brakes, right?


The air pressure in air brakes actually releases a spring which holds the brakes on at all times. The air pressure is regulated by the tow vehicles brake pedal which is really just an air valve. An air actuator pushes against a spring which is holding the brakes on. This assures that in the event that the trailer becomes disconnected its brakes will automatically come on.

Semi Trailer Brakes emergency lines red blue

Every semi trailer has two air lines one is red and one is blue. These are the service line (blue) and emergency line (red). They attach from the truck to the trailer by a quick connector called a Glad-hand. The service air line sometimes called the control line will carry air that is controlled by the foot brake or valve in the truck. The service line activating is the hissing noise you hear in traffic along side a big truck as the driver presses the brake pedal.

The emergency air line supplies the trailer air tanks and will control the emergency brakes on the semi trailer. Loss of air pressure in this line will cause the semi trailer emergency brakes to come on in the event that the trailer disconnects from the truck, or the hose is torn apart. When the emergency line loses pressure it will cause the tractor protection valve to pop out in the truck cab. This allows the truck to not loose air pressure and notifies the driver that he has a serious brake problem in the trailer.

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