How Trailer Brakes Work
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Types
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There are two basic types of brakes used on trailers, but there are three possible means of actuating them. Trailer brakes are either disc or drum brakes, and can be actuate by air, hydraulics or electricity.
Air Brakes
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Air-actuated brakes are found only on large trailers used in semi-trailer rigs. These trailers must be towed by a truck which is itself equipped with air brakes. There are hoses to supply air to the trailer brakes located on the rear of the truck. The brake is actuated when the brake pedal in the tow vehicle is depressed, or when the trailer brake lever is utilized. This lever is mounted on the dash of air brake-equipped trucks and is used to apply the trailer brake independently of the truck's brake.
Air brakes work by applying high pressure air to a diaphragm inside the brake can. This causes the brake rod to apply pressure to the brake shoe actuating rod, which applies the brake -
Surge Brakes
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Trailers equipped with surge brakes do not require a specially equipped tow vehicle. Surge brakes are entirely contained on the trailer, allowing the use of this type of trailer with any vehicle of adequate size and capacity.
Surge brakes are hydraulic brakes and work exactly the same as the brakes in a car. The brake fluid is contained in the brake master cylinder and is sent to the wheels when the piston in the master cylinder is pushed in, usually by means of a brake pedal. However, on a trailer, the master cylinder is mounted to the trailer coupler which is designed to allow the coupler so slide when the tow vehicle is slowing faster than the trailer. This movement is enough to depress the piston in the master cylinder, which sends brake fluid to the brakes. Inside each wheel's brake assembly in a small hydraulic cylinder, called a wheel cylinder. When the brake fluid from the master cylinder enters the wheel cylinder the pistons in the cylinder move outward, spreading the brake shoes, forcing them against the brake drum and slowing the trailer. Surge brakes can be either drum or disc brakes.
Electric Brakes
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Electric trailer brakes require a tow vehicle to be equipped with a brake controller. This type of brake is always a drum brake. The shoes are pressed against the drum because the magnet tries to attach itself to the brake drum, forcing the actuating arm to spread the brake shoes against the drum, which stops the trailer. This type of actuating mechanism is used on all trailer types, with the exception of most boat trailers.
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