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Trailer Brake Types, Pros & Cons

Electric, Surge, Air Brake Types and How They Work

Trailer brake requirements vary State to State, with the most common laws requiring an auxiliary brake system on trailers weighing more than 3,000 lbs. Electric, hydraulic surge, and air brakes are three commonly available trailer brake systems. The type of brake system utilized depends on the application and overall weight of the vehicle. While electric brakes are extremely common, surge brakes are required in most marine applications. Additionally, laws in most jurisdictions require tractor-trailers and other high GCWR commercial vehicles to employ air brake systems.

How Electric Trailer Brakes Work

Electric trailer brakes use voltage from the tow vehicle's battery(ies) to apply the brakes. The system relies on the principle that magnetic force is proportional to the voltage supplied to an electromagnet. An electromagnet located inside each brake drum engages the brake shoes against the drum via a simple lever system. The magnet, attracted to the iron brake drum, moves towards the drum as voltage is supplied - the greater the voltage, the greater the distance that magnet travels and the more powerful the braking force.

The voltage supplied to a each electromagnetic actuator and therefore total braking effort is controlled by a brake controller, a small electronic device that is typically mounted on, in, or around the dash of the vehicle. There are two basic types of brake controllers:

Time Delayed brake controllers apply the brakes with a preset intensity and rate of application. That is, the maximum voltage supplied to each electromagnetic actuator only depends on the driver's preset amount. There is a delay in the time from which the brake pedal is depressed and the trailer brakes reach the full preset braking force. Braking force is therefore increased gradually over a period of time before maximum voltage output is realized. Both the length of the delay and the maximum output voltage is adjustable.

Proportional brake controllers, which include inertia type controllers, sense the rate of deceleration and automatically adjust the voltage output to provide braking effort that corresponds with this rate. The more aggressive the tow vehicle's braking effort, the more the trailer brakes will be applied. As such, these trailer brake controllers provide trailer braking proportional to that of the tow vehicle. Initial braking effort and the aggressiveness of the trailer brake application schedule is adjustable on proportional type controllers.

Trailers with electric brake systems require a standalone battery supply in order to stop the trailer in the event of a breakaway condition, as the tow vehicle loses the ability to apply the trailer brakes once the connection between the tow vehicle and trailer is broken. A breakaway cable is standard on all electric trailer brake systems, which initiates maximum braking effort by means of a tongue mounted switch when the cable is pulled. On tandem axle trailers, brakes are installed on one or both of the trailer axles depending on the trailer's weight rating.

Electric Brake Pros

Electric Brake Cons

Versatile applications

Requires brake controller and experience in adjusting brake controller to match trailer weight

Smooth braking effort when controller is properly adjusted

Trailer requires standalone power supply for breakaway system

Low maintenance system

Trailer cannot be submerged (i.e. marine applications)

How Hydraulic Surge Brakes Work

Hydraulic surge brakes, typically referred to simply as surge brakes, are common on boat and rental trailers. In many instances, they are favorable in applications where a driver may be unfamiliar with electric brake controls or otherwise lack the equipment necessary to operate an electric trailer brake system. In the case of boat trailers, surge brakes are extremely common as the brake system periodically becomes submerged in water when boats are launched and retrieved, which may prematurely wear and/or corrode the electric components housed within the brake drum. In addition, failure to remove the trailer connector prior to submerging the trailer may cause an electrical short, disabling the brake system and prompting a possible safety hazard.

A surge brake system is relatively simple, the heart of which is a master cylinder that operates similar to that found in cars and trucks. The ball coupler is free to slide within the frame or tongue of the trailer where its movement operates the brake master cylinder. As the coupler slides inwards into the tongue, the master cylinder piston increases fluid pressure in the brake lines; greater pressure equates to greater braking force. A spring or springs positioned between the coupler and master cylinder control the application of pressure and maintain that the trailer braking force is roughly proportional to the tow vehicle's braking efforts. Pressurized brake fluid is distributed to each brake drum/rotor by means of a network of individual brake lines.

surge brake operation
Typical hydraulic surge brake system

During braking, the surge brake system is perpetually self adjusting in that trailer braking force is dependent on the braking force of the tow vehicle. When the tow vehicle begins to brake, the trailer attempts to push the tow vehicle, thus applying the trailer brakes until the trailer braking force ideally matches that of the tow vehicle. In practice, surge brakes tend to apply roughly with a noticeable delay and cause "jerky" movement of the tow vehicle until braking force between the truck and trailer reach an equilibrium state. In the situation of a trailer break away, where the trailer coupler becomes detached from the tow vehicle, a safety cable actuates the master cylinder and applies the brakes so that the trailer comes to a halt. The obvious hardship faced by surge brake systems is in the backing of a trailer. As the vehicle moves in reverse, the brakes will have a tendency to apply. The solution is to pin the coupler assembly to the tongue so that it cannot slide and actuate the master cylinder.

Surge Brake Pros

Surge Brake Cons

No electric controller or special wiring necessary

Trailer coupler needs to be pinned to the tongue to eliminate brake engagement prior to backing

No setup or knowledge of braking system necessary - hitch and go

Jerky, rough braking effort

Trailer can be submerged with electrical connection to tow vehicle unplugged

Periodic maintenance required

How Air Brakes Work

Air brake systems rely on a supply of pressurized air to engage and disengage both the tow vehicle and trailer brakes. Air pressure is produced via an engine mounted compressor, which maintains pressure within a tank or reservoir. For trailers, a spring loaded air brake chamber - a type of actuator - located at each wheel controls the movement of the individual brake shoes. Trailer air brake systems are unique in that pressure is released, not increased, as the brake pedal is depressed. The release of pressure to each individual brake chamber activates movement of the brake pads/shoes. This is due to the fact that each trailer brake chamber is spring loaded and defaults to the applied position (full braking effort) at 0 lbs of pressure. As pressure to the brake chamber increases, the spring depresses and braking force reduces proportionally. It typically requires roughly 60 psi of air pressure to fully release the trailer brakes, while 120 to 140 psi maximum pressure is typically stored at all times.

Air brakes are the only trailer braking system that will bring the vehicle to a stop if the system's working fluid should develop a leak and/or loses pressure. Should air pressure to one or more brake chamber abruptly drop, the spring in the air brake chamber will lockup and bring the vehicle to a stop. For comparison, consider a severe fluid leak in a hydraulic brake system - the brakes lose stopping power and are unable to be applied; the opposite is true for air brakes. Both electric and surge brake systems require a trailer breakaway in order to fully apply the trailer brakes, while air brakes do not. In a breakaway situation for a trailer equipped with air brakes, the brake chambers would immediately lose pressure and the brakes would lockup.

Air Brake Pros

Air Brake Cons

Air supply is limitless and pressure is constantly maintained

Special training and licensing is required in most jurisdictions in order to drive air brake equipped vehicles


Bleeding of brake lines not required

Brake operation persists even when system experiences minor leaks

Requires frequent maintenance, service, inspection, and monitoring


Brakes default to applied position when pressure is lost do to failure

Water vapor captured in the air supply can freeze in cold weather (many preventative measures can be taken)