* Adjusted too tight: The star adjuster wheel, which controls the distance between the brake shoes and drum, may be turned too far in. This is a very common cause. The shoes are constantly pressing against the drum, causing the grabbing.
* Worn or damaged brake shoes: Worn brake shoes are uneven, leading to uneven braking and grabbing. Damage (cracks, scoring) can also cause the shoes to bind.
* Seized wheel cylinder: The wheel cylinder pushes the brake shoes outward. If it seizes (doesn't retract properly), it will keep the brakes applied. This is a serious problem that often requires replacement of the wheel cylinder.
* Damaged or contaminated brake drums: A drum that's warped, scored, or rusty will prevent smooth brake shoe retraction. Contaminants (like grease or oil) can also cause the shoes to stick.
* Sticky or corroded brake hardware: The return springs, pins, and other hardware that allow the brake shoes to retract can become corroded, sticky, or broken, preventing proper release.
* Low-quality brake parts: Using substandard replacement parts can lead to poor performance and premature failure.
* Hydraulic issues: While less likely, a problem with the hydraulic system, such as a leak or malfunctioning master cylinder, could contribute to a constant application of brake pressure. This is less likely to cause grabbing in *only* the rear drums.
What to do:
1. Safety First: If the brakes are grabbing significantly, driving the truck is unsafe. Have it towed to a mechanic or a safe location for repair.
2. Visual Inspection: Start by visually inspecting the rear brakes. Look for obvious signs of wear, damage, corrosion, or contamination. Check the brake drums for scoring, warping, and rust. Inspect the brake shoes, springs, and hardware. Check the wheel cylinders for leaks.
3. Wheel Cylinder Test: You can try manually pushing the wheel cylinder pistons back in. If they don't move freely or spring back, it likely needs replacement. (Note: Be careful, brake fluid is messy.)
4. Professional Diagnosis: Unless you have significant experience working on brakes, it's best to take the truck to a qualified mechanic for a proper diagnosis and repair. Incorrectly adjusted brakes are dangerous.
The mechanic will be able to accurately identify the cause of the grabbing and perform the necessary repairs, which might involve replacing shoes, drums, wheel cylinders, hardware, or even a master cylinder depending on the diagnosis. Don't attempt repairs beyond your skill level.
