How to Adjust Self-Adjusting Trailer Drum Brakes

To adjust self-adjusting trailer drum brakes, set an initial “preload” by turning the star wheel through the backing-plate slot until the drum shows a light, even drag, then perform a series of firm, low-speed stops to let the mechanism finish the adjustment. While these brakes tune themselves as you drive, they still need a correct baseline and periodic checks to ensure even, confident stopping. Here’s how to do it safely and correctly for electric or hydraulic surge drum systems.

Know Your Braking System

Before you begin, identify what you have. Most modern trailers use electric drum brakes with built-in self-adjusters (e.g., common Dexter or Lippert assemblies). Some boat and utility trailers use hydraulic surge drum brakes with self-adjusters. The basic setup and star-wheel adjustment are similar, but the road-test step differs slightly depending on whether the adjusters index during forward or reverse stops.

The points below outline the common types you may encounter and why it matters for procedure.

• Electric self-adjusting drum brakes: The adjuster advances incrementally during firm stops, typically in forward motion on many modern designs.
• Hydraulic surge drum brakes (self-adjusting): The wheel cylinder action and adjuster lever advance the star wheel under braking; some older mechanisms rely on reverse braking to trigger adjustment.
• Manual-adjust drum brakes: If yours lacks the adjuster lever and spring, it is not self-adjusting and must be periodically adjusted by hand only.

Confirming your type ensures you perform the correct road-test sequence (forward stops, reverse stops, or both) and set expectations for ongoing maintenance.

Tools and Safety Prep

Gather the right tools and make the work area safe. Proper support and clean access to the backing plate are essential to avoid injury and to achieve an accurate initial setting.

• Wheel chocks, floor jack rated for the trailer, and jack stands placed under axle tubes near the spring seats or manufacturer-approved points.
• Brake adjusting tool (brake “spoon”) or a flat screwdriver, flashlight, and needle-nose pliers.
• Torque wrench for lug nuts and hand tools to remove dust caps and plugs.
• Infared thermometer (optional but helpful) for temperature comparisons after the road test.
• Gloves and eye protection; shop rags and brake cleaner to keep friction surfaces clean.

Take a moment to set wheel chocks, work on level ground, keep the trailer unplugged from the tow vehicle (no power to magnets), and ensure the breakaway switch pin is properly installed so brakes aren’t inadvertently energized.

Initial Manual Preset (with Wheels On)

Self-adjusting brakes still need a baseline. This initial manual preset brings the shoes close to the drum so the self-adjuster can make fine, consistent increments during your test drive.

1. Chock the opposite wheels. Jack up one wheel and support the axle with a stand. The wheel you’re adjusting must spin freely.
2. On the backing plate, pry out the rubber access plug at the bottom to expose the star wheel.
3. Insert the brake spoon and rotate the star wheel to expand the shoes. Direction varies by side; if the wheel tightens and locks after a few clicks, you’re expanding. If it loosens, you’re retracting—reverse direction.
4. Turn the star wheel until the drum locks or becomes very hard to turn by hand.
5. Back the star wheel off a few clicks at a time until the wheel spins with a light, even scrape all the way around—typically about 3/4 to 1 revolution on a good push with steady, slight drag.
6. Repeat for each wheel end. Replace the rubber access plugs when done.

This slight, uniform drag is your target. It centers the shoes and provides the self-adjuster a consistent starting point across the axle(s).

Let the Self-Adjusters Finish the Job (Road Test)

Once preloaded, the adjuster mechanism will fine-tune shoe clearance under braking. Most modern trailer self-adjusters index during firm forward stops, while some older styles adjust during reverse stops. Doing both forward and a few reverse stops covers either design.

1. Lower the trailer, torque lug nuts to the wheel-stud specification (consult your axle/wheel maker), and move to a safe, empty lot.
2. Make 6–10 firm, straight-line stops from about 20–25 mph down to ~5–10 mph. Do not lock the wheels; aim for solid deceleration that loads the brakes.
3. If you’re unsure of your adjuster’s direction, add 3–5 firm reverse stops at walking speed in a straight line in a clear area.
4. Allow brief cool-downs between groups of stops to avoid overheating the linings.
5. Repark, re-chock, and recheck wheel drag off the ground. You’re looking for even, light drag side-to-side. Use an IR thermometer to compare hub temperatures after a short drive; they should be similar across an axle.

The adjusters should now be evenly indexed. Uneven drag or temperature suggests one side needs a small manual tweak or further inspection.

Bed-In the Linings

Fresh or newly adjusted brakes perform best after a controlled break-in. Bedding smooths the friction surfaces and stabilizes response.

1. Perform 15–30 moderate stops from ~25 mph to ~10 mph, allowing 30–60 seconds of rolling or idle time between stops for cooling.
2. Avoid holding the brakes while stopped immediately after a hard application, which can imprint pads/shoes.
3. After bedding, recheck for slight, even drag and verify hub temperatures remain consistent side-to-side.

Proper bedding reduces noise, evens out adjustment rates, and improves initial bite and stability under load.

Troubleshooting and Fine-Tuning

If the brakes don’t feel balanced or one side runs hotter, a few quick checks can isolate the issue and prevent premature wear.

• Uneven drag or heat: Back off the hotter side 2–3 clicks, or advance the cooler side 2–3 clicks; re-road-test and recheck.
• Adjuster not advancing: Inspect the self-adjuster lever, spring, and star wheel for binding, corrosion, or incorrect assembly; clean and lightly lubricate star-wheel threads with high-temp brake lubricant (keep off linings).
• Orientation errors: Left- and right-hand brake assemblies are side-specific; swapped assemblies can defeat self-adjusting action.
• Contamination or glazing: Oil or grease on shoes/drum, or glazed linings, will impair adjustment. Replace contaminated shoes and fix any leaking grease seals; deglaze drums if needed.
• Electric brake specifics: Verify good ground, proper controller gain and boost, and healthy magnets/wiring. A weak magnet reduces adjustment force.
• Hydraulic surge specifics: Ensure the actuator slides freely and the reverse lockout (if equipped) functions so reverse stops can occur when needed.
• Bearing play: Excessive hub endplay changes shoe-to-drum clearance. Service bearings and set proper preload before adjusting brakes.

Addressing these items typically restores symmetrical braking and reliable self-adjustment, extending shoe life and improving control.

Service Intervals and Specs

Even self-adjusters need regular attention. Periodic checks keep the mechanism free and the friction pair healthy.

• Inspection: Every 3,000–5,000 miles or quarterly for frequent use; before long trips; after immersion (boat trailers).
• Star-wheel access plug: Keep the rubber plugs installed to block dust and water ingress.
• Bearing service: Clean, inspect, repack, and set preload annually or per manufacturer guidance; replace seals if any leakage is found.
• Lug torque: Always torque wheels to manufacturer specs after service and recheck after 25–50 miles.
• Replacement thresholds: Replace shoes that are oil-soaked, cracked, or worn near the rivets/bond line; replace drums out of round or beyond max diameter.

Following these intervals keeps the self-adjuster free to do its job and protects the brake hardware from accelerated wear.

Common Mistakes to Avoid

Small missteps can leave brakes weak, grabby, or uneven. Watch for these frequent errors.

• Setting zero drag: Leaving the shoes too loose prevents the adjuster from taking up slack efficiently.
• Overtightening: Excessive drag overheats brakes and can glaze shoes; aim for a light, uniform scrape.
• Ignoring side-specific parts: Mixing left/right assemblies or adjuster hardware stops self-adjustment.
• Contaminating friction surfaces: Keep grease and lubricant off shoes and drum faces.
• Skipping torque checks: Loose or over-torqued wheels jeopardize safety and brake performance.

A careful setup and a few quick verifications help you avoid rework and ensure consistent stopping power.

Summary

Set a light, even shoe drag through the backing-plate slot, then perform a series of firm, controlled stops to let the mechanism self-adjust—primarily in forward motion on most modern systems, with a few reverse stops if needed. Recheck drag and hub temperatures for balance, bed the linings, and maintain the system with periodic inspections. Done right, self-adjusting trailer drum brakes deliver stable, even braking with minimal ongoing manual adjustment.

How do self-adjusting trailer brakes adjust?

Forward Self-Adjusting Brakes have a mechanism that automatically adjusts the brake shoes during every forward stop. A cable attached to the front brake shoe wraps around a pulley on the rear brake shoe.

How to tell if brakes are self-adjusting?

Visual Identification
To check if brakes are self-adjusting without removing the hub, remove the adjuster slot plugs on the backside of the brakes (Fig. 1A) and shine a flashlight in the holes. If the brakes are self-adjusting, the silver lever (Fig. 2A) that rotates the star lever will be visible.

How do drum brakes self-adjust?

Drum brakes self-adjust through a mechanical system of a star wheel adjuster, a lever, and a cable that senses slack from brake shoe wear and rotates the star wheel to move the shoes closer to the drum. This adjustment typically occurs when the vehicle is put in reverse and the brakes are applied, or when the parking brake is engaged. The movement of the brake shoes in reverse allows the adjuster cable to pull a pawl, which “walks” up the teeth of the star wheel, tightening the shoes to maintain optimal clearance.
 
You can watch this video to see a diagram of how drum brakes self-adjust: 57sThomas EXOVCDSYouTube · Sep 22, 2019
How the Self-Adjuster Works

1. Wear Detection: As brake shoe linings wear down, the space between the shoes and the drum increases. 
2. Reverse Activation: When the vehicle is in reverse and the brakes are applied, the brake shoes rotate slightly in the reverse direction. 
3. Lever and Cable: This movement causes a linkage mechanism (a lever and cable) to activate. 
4. Star Wheel Engagement: The lever moves, and the attached cable (or pawl) engages the notches on the star wheel. 
5. Ratchet Action: The pawl “skips” up the teeth of the star wheel, rotating it to expand the brake shoes slightly. 
6. Tighter Shoes: This process increases the diameter of the brake shoes, bringing them closer to the drum to compensate for the wear. 

Why Reverse is Important

• The self-adjuster mechanism is designed to operate when the drum’s rotation pulls the shoes inward. In the forward direction, the shoes are dragged away from the drum, preventing the adjuster from working. 
• Activating the adjuster in reverse allows for the necessary “slack” to be taken up by the mechanism. 

When to Activate the Self-Adjuster

• To test and activate the self-adjuster, drive in reverse and stop abruptly a few times. 
• Setting and releasing the parking brake several times can also activate the self-adjusting mechanism. 

How do self-adjusting brake calipers work?

The single-piston floating-caliper disc brake is self-centering and self-adjusting. The caliper is able to slide from side to side so it will move to the center each time the brakes are applied.