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Monday, October 26, 2009

Electric Brake Lock-Up

The right front brake on my travel trailer has locked up on three occasions. It always happens when I am leaving a gas station and turning right and applying the brakes during the turn. The brake locks and will only release if I back up a little. The first time it happened it shredded the tire before I realized what was happening.
Don, (Downer’s Grove, IL)


Don, it is quite possible there is a mechanical problem with some of the components in that brake assembly. But you cannot rule out an electrical problem either. In either case, it will be necessary to remove the tire and wheel and closely inspect the various brake components. Inspect the magnet arm for any loose or worn parts. Check the brake shoe return springs, mounting springs, and adjuster spring for deformation. Manually move the actuating arm and check for any binding or rubbing. Replace the magnets if they show signs of uneven wear.

Though not all wheel assemblies have a replaceable armature, it may be necessary in some instances to have that plate resurfaced. This cannot be done without a special lathe. Call your local service facility if the armature has developed a pattern of grooves due to worn or damaged magnets. In newer drum assemblies it will be necessary to replace the complete unit.

It is wise to replace all the same items of the same axle even if only one side necessitates replacing. If, for instance the right side magnet is worn, but the left side is okay, replace them both. Keep all like components on the same axle, the same age.

There are two areas of the brake drum that are subject to wear and require periodic inspection. These two areas are the drum surface where the brake shoes make contact during stopping and the aforementioned armature surface where the magnet contacts. The drum surface should be inspected for excessive wear or heavy scoring. If the wear marks are worn more than .020 inches or the drum has worn out of round by more than .015 inches, then the drum surface should be turned. If the scoring is greater than .090 inches, the drum must be replaced.

To ensure proper contact between the armature face and the magnet face, the magnets should be replaced whenever the armature place is resurfaced. Unlike most automotive brakes which are self-adjusting, electric brakes require periodic adjustment to keep the shoes properly spaced inside the drum. Here is the procedure: jack up and properly support the trailer so that the weight is off the wheel and remove the plug from the back side of the backing plate so that an adjusting tool can be inserted through the hole. Engage the star wheel (see photo) and rotate it to expand the shoes against the drum (rotation may be clockwise or counter-clockwise, depending on the brake manufacturer). Expand the shoes until you cannot rotate the tire at all. This centers or seats each shoe evenly against the surface of the drum. Then back the adjustment off in the opposite direction until the tire turns freely yet has a very slight drag while rotating it by hand.

Repeat this process on each brake. If all the components appear to be in working order and nothing appears to be bent or damaged, at the very least, replace all the springs in all four brake assemblies. But it’s my guess you’ll find a broken spring or a damaged or bent component somewhere in there.

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