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Trailer Brakes: Electric vs. Hydraulic Surge

John Tiger, Jr.
Speedway Illustrated Magazine
September 2003

Trailer brakes are a necessary evil; when they work properly, they're an essential part of the installed towing equipment needed to make a trip safe; when they need work, they're a real pain in the %&^* to service correctly. Most states have towing laws that stipulate that trailer brakes (separate from tow vehicle brakes) are mandatory when the trailer exceeds a certain weight limit; most times, that limit is around 3000 pounds (although in some states it's 1500 pounds, and in others it's 4500 pounds). Your state's information can usually be found at government Web sites such as the Department of Transportation (DOT; www.dot.gov) or the National Highway Traffic & Safety Administration (NHTSA; www.nhtsa.gov). Additionally, towing-related publications such as Trailer Boats magazine (www.trailerboats.com) or Trailer Life magazine (www.trailerlife.com) regularly print very helpful state-by-state towing laws summary each fall, as well as a Tow Ratings guide for the new vehicle model year. Many new vehicle owners' manuals require trailer brakes when towing in order to keep the warranty valid. If your vehicle is new or under warranty, check the manual before you tow.


Trailer brakes fall into two categories; electric (controlled by a brake control in the tow vehicle) and hydraulic surge (actuated by a special trailer coupler with no control from the tow car). Typically, hydraulic surge brakes are fitted to boat trailers and rental utility trailers.

In the past, boat trailers relied heavily on surge brakes setups because it was thought that the electric brake components mounted in the wheel (the shoes, arms, magnet and related springs and parts) would rust quickly because they are constantly dipped in water when the boat is launched. Today, however, more marine trailer builders are installing electric brake systems because brake manufacturers have started offering corrosion-resistant brake components such as galvanized or stainless steel metal parts coupled with rare-earth magnets.

Rental trailers, like those from U-Haul, rely on surge brakes because they don't require a brake control and related wiring from the tow vehicle. This makes renting and hooking up the trailer easier and cheaper.


However, there have always been questions about the actual legality of surge brake systems. DOT regulations specify that trailers with brakes must be fitted with an actuator that allows the tow vehicle driver to operate the trailer brakes independent of the tow vehicle brakes. In other words, he must be able to actuate the trailer's brakes without stepping on the tow vehicle brake pedal. Surge brakes do not offer this feature. They work using the deceleration force present as the tow vehicle stops. When the driver applies the tow vehicle brakes, the surge brake coupler's internal master cylinder compresses against the coupler body, forcing brake fluid through the brake lines to the wheel cylinders which forces the brake shoes against the drum (or pads against the rotor, if equipped with the newer disc brakes). If this sounds like a description of how the tow vehicle's brakes work, that's because surge brake systems work very much like car and truck brake systems. Unfortunately, there's no way for the driver to independently apply the trailer brakes in case of emergency. Are surge brakes legal? Technically, no — but that's a "technicality" that's been overlooked for decades. Surge brakes are still very popular on marine and rental trailers, and probably will continue to be for years into the future.

Surge brake maintenance can be time-consuming and troublesome. Just like the tow vehicle's brakes, the trailer's brakes must be maintained and serviced regularly to ensure that they'll work properly when they're needed most. With surge brakes, this involves changing the brake fluid, checking and/or replacing the lines and fittings carefully when corroded or leaking, and replacing the brake shoes and related parts. In addition, just like when servicing tow vehicle brakes, surge brakes must be bled in order to work properly. It's no wonder most surge brake systems go unserviced for many years, sometimes for the entire life of the trailer if it's used infrequently.


Electric trailer brakes work without hydraulic fluid, master cylinders, or brake lines. An electric brake controller is mounted in the tow vehicle (usually under the dashboard, within easy reach of the driver). This controller is a simple device that takes 12 volts DC from the tow vehicle's electrical system and sends it back to the trailer brakes through a simple wiring system. The brake controller is always powered (always "on") as it is tied directly into the tow vehicle's wiring. However, it is "triggered" (energized) and begins to send power back to the trailer brakes only when activated. It can be activated two ways. Since it is wired directly into the tow vehicle's brake light switch, when the driver steps on the tow vehicle brake pedal he also activates the brake control. In addition, all brake controls have a manual actuation lever or button that allows the driver to send power back to the trailer brakes without stepping on the tow vehicle brake pedal.

Most brake controls employ some type of internal electronic control whereby the 12-volt input from the tow vehicle's electrical system is modulated as it's sent back to the trailer brakes. There are two types of brake controls: inertia-activated and time activated. Both have their advantages and disadvantages, but they perform essentially the same function: they allow voltage to be applied to the trailer brake system wiring, energizing the trailer brakes so that the trailer helps the tow vehicle slow or stop.

Electric trailer brakes employ a magnet mounted inside the wheel hub assembly that when energized by the brake control, causes the brake shoes to move outward towards the drum and push against it. In contrast to surge brakes, servicing electric brakes is relatively easy; the magnet, wires, brake shoes and return springs are the only parts to service or replace, and there's no hydraulic fluid to replace and bleed. There's no master cylinder or lines to leak either.


For most Speedway Illustrated readers, the choice will be easy; if you're fitting out a new trailer or rebuilding an older one, go with electric brakes. Surge brakes work well, but they're harder and more complex to maintain and repair. Electric brakes are easier to install and maintain, and the only extra initial expense is the brake control. There's good news for those in need of one, though; prices have come down over the past half-dozen years due to more competition in this market. Good brake controls that will stop trailers with up to four brakes can be purchased for less than $75, and on most new trucks and SUVs they simply plug into an existing harness under the dash using an inexpensive (less than $20) harness adapter. Remember that a surge brake system uses a special trailer coupler with a master cylinder included, which can cost well over $100 when buying new or replacing an older unit. As mentioned, maintenance and upkeep are much simpler than with surge brakes, and you'll have the added safety and peace of mind that a cockpit-mounted brake control provides.

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