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How to overhaul your trailer brakes

In this instalment of Shop Class, we inspect and repair the brakes on an old livestock trailer

In this instalment of Shop Class, we tackle a wheel hub inspection and electric brake service on an old livestock trailer.

Most provinces require all trailers with a gross weight of more than 950 kg (2,000 pounds) to be equipped with working brakes. Along with extra stopping power, they provide stability when braking, especially during hard brake applications and on slippery roads. So pulling a loaded trailer without them is just plain dangerous. And like any other mechanical system, they need regular maintenance to stay in good working condition. Here’s a look at what you’ll find when you pull the wheel hubs off, and what you need to consider doing to keep trailer brakes in good condition.

First, you need to understand how they work. Electric trailer brakes use a slightly different mechanism than you’ll find on a car or truck. When electrical current runs to the brakes from the towing vehicle, that causes an electromagnet in each trailer hub to slide outward along a short shaft and attract itself to the inside of the rotating drum. As the drum turns, it pushes the magnet toward the rear of the trailer. The magnet is connected to a lever arm which turns an eccentric forcing the brake shoes outward into contact with the drum providing friction and, therefore, braking.

Because the magnet also comes into contact with the drum, there is an additional wear surface on the drum, one on the face where the magnet rubs against it and the other where the shoes touch. And the magnet is subject to wear, just like the drum.

To inspect and repair trailer brakes, first pry off the dust cap in the centre of the hub that covers the axle nut. Remove the cotter pin and nut, which allows the drum to slide off the axle. Be prepared to catch the outer bearing, which will fall out from behind the nut as you slide the drum off.

Inspect the inside of the drum and ensure the wear surfaces where the magnet and shoes contact it are smooth and there isn’t excessive wear. Drums can be resurfaced on a special lathe at a brake shop if there is enough thickness remaining. If not and there are wear grooves, you’ll need to replace it. The shoes should be changed if you replace or resurface the drums.

Check the shoes for material thickness, too. If the surfaces are worn down close to the rivet heads, replace them. Check the magnet for wear. On the face of new magnets are four shallow holes that serve as wear indicators. If they are no longer visible, replace the magnet. Apply special braking system grease to the bar the magnet slides on and the lever pivot point. Don’t use regular grease.

If there is a lot of wear on the brakes, simply replacing the entire backing plate assembly and fitting a new drum may be the cheapest and easiest option.

Inspect the two wheel bearings, the inner and outer, and repack them with fresh grease. (You can use ordinary grease here.) To get at the inner bearing, you’ll have to pry off the grease seal, which will have to be replaced with a new one. To repack the bearings, put a large lump of grease in the palm of your hand and push the wide edge of the bearing into it repeatedly until fresh grease oozes out the top. Do this around the entire bearing.

A handy tip when replacing the grease seal is to use a short piece of 2×4. Put the seal in position and place the 2×4 over top of it. Hit the 2×4 with a hammer to force the seal into position evenly, without damaging it. Seat the seal flush with the surface of the drum.

Don’t over tighten the axle nut when reinstalling the drum. Start by spinning the drum by hand as you tighten the nut to where it just stops the drum from turning, then back the nut off until the drum turns freely. That seats the bearings. The sweet spot for securing the axle nut is where the drum turns freely but has no free play, meaning you can’t wiggle it on the axle.

Remember to adjust the shoes after installing the drum. That is done by sticking a screwdriver (or special brake tool) through a slot in the rear of the backing plate and turning the adjuster mechanism. Turn the adjuster until the shoes touch the drum, then back them off about five or six notches. And the rule of thumb on brake repairs is whatever you do to one hub, you should do to its partner on the other side of the axle to ensure even braking on both sides.

About the author


Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.



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