How to unlock trailer brakes to back up

Updated April 17, 2017

Trailer brakes on fifth-wheel, boat trailers and travel trailers have a lot in common with standard automobile brakes in that the components that stop the vehicles function in the same manner. Trailer brakes come in surge and electric configurations, and either one can cause a failure in the brake systems, as well as a failure arising from the brake components themselves. Locked trailer brakes can result from a number of causes, and narrowing down the failed component can be performed by the trailer owner if he knows what to look for and how to remedy the problem.

Place your tow vehicle in park or neutral with the emergency brake firmly set. Use a tire iron to loosen the lug nuts on one trailer wheel, but do not remove them. Raise one side of the trailer next to the wheel and place a jack stand under the frame. Finish removing the lug nuts and pull the wheel free. Remove the hub dust cover with a screwdriver. Use pliers to remove the cottar pin on the castellated nut. Remove the castellated nut with the pliers and pull the drum off.

Inspect the brake lining and drum for deep gouges and grooves. Make sure the hold-down and return spring sit in their proper positions. Push the tops of the brakes shoes together to see if they compress. If they do not compress, you have a stuck wheel cylinder that binds the shoes against the drum. Use pliers to grip and shove the wheel cylinder piston rods back and forth on both sides to free them up.

Turn the small adjusting star that sits between the bottom of the brake shoes with your pliers. Turn it clockwise to bring the shoes inward. Replace the drum and screw the castellated nut back on with pliers until the drum is snug. Replace the cottar pin in the nut and flare the ends with pliers. Spin the drum by hand. You should feel a slight drag on it. To adjust it, stick a flathead screwdriver through the rear of the back plating to connect with the adjusting star gear.

Cock your wrist up and down to adjust the star, and spin the drum with the other hand until you feel a perceptible drag on the drum. This will be your drum brake adjustment. Replace the wheel on the hub and screw the lug nuts on with the tire iron. Use the floor jack to lift the frame and remove the jack stand. Use a torque wrench to tighten the wheel lugs nuts according to your owner's manual specifications. Perform the same procedure on the opposite wheel of the trailer.

Review your owner's manual to determine if you have the addition of surge or electric brakes on your trailer. If you have electric brakes, you have an inertia switch, usually mounted on the dashboard, which controls a magnetic actuator that controls your brakes. The inertia control switch has an adjustment on it. Make sure it has not been adjusted for am overly heavy load, causing a lock-up. Check the wire jack connector at the trailer tongue, and clean the connectors with some solvent and Q-tips.

Look for the surge coupler on the trailer tongue. It has a small master cylinder with a linkage arm inside it. Remove the cap on the master cylinder and check the brake fluid level. Replace the cap. Have your assistant step on the trailer brakes while you use a bleeder wrench to loosen the bleeder valve on one of the trailer brake wheel cylinders. Use the floor jack if you need clearance underneath.

Turn the bleeder valve counterclockwise. If a steady stream of brake fluid comes out under pressure, you have a bad trailer master cylinder and actuator, which will cause the brakes to lock up. Replace the master cylinder and actuator.

Look for the emergency chain or cable disconnect on your trailer, if so equipped. The device automatically locks up the brakes if the trailer becomes disconnected from the tow vehicle while driving. The cable or chain should be joined into its sockets between the tow vehicle and trailer at both ends. Disconnect and clean both connector ends with solvent and Q-tips. Reinstall it.

Refer to your trailer owner's manual for the location of the reverse solenoid wire that connects from the trailer to the reversing lights on your tow vehicle. The solenoid disengages the brakes when the reversing lights activate. Check the wire connections at both ends for loose or broken fittings. Clean the connectors with solvent and Q-tips. Check the fuse for the reversing lights to make sure they function properly.


Check your brake components at the wheel first. If you have electric brakes, have an assistant step on the trailer brake while you have the drum off the hub. Watch for the magnetic actuator to open the brake shoes. If it does not, the actuator is bad. Check your owner's manual to confirm whether you have surge brakes.

Things You'll Need

  • Trailer operator's manual
  • Tire iron
  • Floor jack (5-ton commercial)
  • Jack stands
  • Pliers
  • Cotter pins
  • Screwdrivers
  • Torque wrench
  • Cleaning solvent
  • Q-tips
  • Bleeder wrenches
  • Assistant
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About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.