* Parking Brake Engaged: This is the simplest and most common cause. Make absolutely sure the parking brake is completely disengaged.
* Brake System Issues (Rear): Problems within the rear brake system itself are a serious possibility:
* Worn or Damaged Brake Pads/Shoes: Severely worn pads or shoes can cause the brakes to grab excessively.
* Frozen or Seized Calipers/Wheel Cylinders: If a caliper or wheel cylinder is seized, it will keep the brake pads pressed against the rotor, even when the brake pedal is released.
* Leaking Wheel Cylinders: This leads to low brake fluid pressure in that wheel and could cause uneven braking and locking.
* Internal Brake System Leaks: A leak in a line or component affecting only the rear brakes.
* Proportioning Valve Issue: While less likely, a malfunctioning proportioning valve (which regulates front-to-rear brake pressure) could be sending too much pressure to the rear brakes during backing up. This is less common but still possible.
* Differential Issues (Less Likely): While less common, a problem within the rear differential could contribute to wheel lockup *indirectly*. For example, a severely damaged differential could create excessive resistance, making it *feel* like the brakes are locked even though they aren't. This would usually be accompanied by other symptoms like loud noises or difficulty turning.
* Tire Pressure Issues (Less Likely): While dramatically unequal tire pressure would affect braking, it's less likely to cause *complete* lockup.
Troubleshooting Steps:
1. Check the Parking Brake: The absolute first thing to do is ensure the parking brake is fully released.
2. Inspect the Rear Brakes: Visually inspect the rear brake pads/shoes for wear and tear. Look for any signs of leaking fluid or seized components.
3. Test the Brake Pedal: Test the brake pedal feel. Does it feel spongy? Is it hard to depress? These could be signs of low brake fluid or other issues.
4. Professional Inspection: If you are unsure about any of the above, you absolutely must take the truck to a qualified mechanic for a thorough brake inspection. Brakes are critical safety components, and attempting to diagnose and repair them yourself without the necessary knowledge and tools is dangerous.
Never drive a vehicle with malfunctioning brakes. If the rear wheels lock up consistently while backing up, the vehicle is unsafe to operate until the problem is professionally diagnosed and repaired.
