How to Adjust Trailer Brakes
Electric drum brakes are adjusted with a star wheel behind the backing plate: jack the wheel clear, turn the star until the drum locks, then back it off until the wheel spins with light drag. Both sides get the same treatment, because uneven adjustment is what makes a trailer pull under braking.
Two different things get called "adjusting the brakes" and confusing them wastes an afternoon. Mechanical adjustment sets how close the shoes sit to the drum, and it is done at the wheel with a screwdriver. Gain sets how hard the controller drives them, and it is done from the cab with a dial. If your trailer brakes feel weak, do the mechanical adjustment first — turning the gain up to compensate for shoes that are too far from the drum just heats everything up.
When adjustment is due
New shoes bed in over the first few hundred miles and will need setting again after that. Beyond that, adjust at the start of a towing season, after any wheel-off work, and whenever you notice the symptoms below. Most modern electric drum brakes are nominally self-adjusting in reverse, but the mechanism relies on firm reverse stops that many trailers rarely see, so they still drift out over time.
What you need
A jack and axle stands rated above the corner weight, a wheel chock for the wheels staying down, a flat-blade screwdriver or a proper brake spoon, and a torque wrench for the lug nuts on reassembly. Nothing else.
The procedure
1. Chock and lift. Chock a wheel that is staying on the ground, then jack the axle and support it on stands. The wheel you are adjusting must spin freely by hand.
2. Find the slot. There is an oblong access slot in the backing plate behind the drum, usually near the bottom. It may be covered by a rubber plug. Behind it sits the star wheel adjuster.
3. Expand until it locks. Put the screwdriver through the slot onto the star wheel and turn it in the direction that expands the shoes, clicking one tooth at a time, until the drum will not rotate at all. Which way that is depends on which side of the trailer you are on and how the backing plate is oriented, so go by what the drum does, not by a remembered direction.
4. Back it off. Reverse the star wheel about ten clicks, until the wheel turns with light, even drag. You should hear a faint continuous brush from the shoes, not a scrape and not silence.
5. Do the other side identically. Count the clicks. The two sides must match — this is the whole point of the exercise, and it is the step people skip.
6. Reassemble and torque. Refit the plug, drop the trailer, and torque the lug nuts to the manufacturer figure in a star pattern. Re-torque after the first 50 miles.
Reading the symptoms
| What you feel | Usual cause |
|---|---|
| Trailer pulls left or right under braking | Shoes adjusted unevenly between sides, or one magnet failing |
| Brakes feel weak everywhere | Shoes too far from the drum, or low gain, or a poor ground |
| Grabbing and locking at low speed | Gain set too high, or adjusted too tight |
| Hot hub after a short run | Adjusted too tight, or a seized adjuster holding the shoes on |
| No braking at all on one side | Electrical, not mechanical — check the ground first |
The last row matters: if one side does nothing, stop adjusting and go to the wiring. A brake that is mechanically perfect does nothing without current. See wiring trailer brakes.
Then set the gain
Mechanical adjustment done, set the controller gain on a loaded trailer — an empty one will always lock early and give you a number that is far too low for real use. At around 25 mph on a clear surface, apply the controller's manual lever alone, without the brake pedal. The trailer should pull the rig up firmly without locking a wheel. Full detail in brake controllers explained.
When to stop and replace
If the star wheel will not turn, if the shoes are down near the rivets, if the drum is scored or blued, or if you cannot get equal drag on both sides at similar click counts, adjustment is not the answer any more. Drum and shoe kits for common 3,500 lb and 7,000 lb axles are inexpensive relative to the consequence of getting this wrong on a loaded dump trailer.
Related questions
After the first few hundred miles on new shoes, then at the start of each towing season and after any wheel-off work. Self-adjusting brakes still drift because the mechanism needs firm reverse stops to operate.
Almost always uneven adjustment between the two sides, or one magnet that has failed. Count the star-wheel clicks on both sides so they match, and if one side still does nothing, check its ground connection.
Yes. They self-adjust when braked in reverse, and most trailers do not reverse under braking often enough for it to keep up. Treat the feature as a slow-down, not a replacement.
Not first. Weak braking is usually shoes sitting too far from the drum, or a bad ground. Raising gain to mask a mechanical problem overheats the brakes and hides the real fault.
