* Proportioning valve malfunction: This valve regulates the braking force sent to the rear wheels. If it's faulty or stuck, it might send too much pressure to the rear, causing them to lock. This is a common cause.
* Excessive rear brake adjustment: If the rear brakes are adjusted too tight (especially on drum brakes), they'll drag and be more likely to lock under braking.
* Low tire pressure (rear): Lower tire pressure reduces the tire's contact patch with the road, increasing the pressure on a smaller area and making it easier for the brakes to lock up.
* Seized caliper/wheel cylinder: A seized caliper (disc brakes) or wheel cylinder (drum brakes) will constantly apply the brakes, leading to locking or severely dragging.
* Parking brake partially engaged: A partially engaged parking brake can contribute to rear wheel lockup, especially at low speeds.
* Brake fluid contamination: Contaminated brake fluid can reduce its effectiveness, leading to an increased need for braking force. The rear brakes, being less powerful than the front, are more susceptible.
* Uneven brake pad/shoe wear: Significant wear differences between the brake pads or shoes on the rear wheels can lead to uneven braking force distribution and lockup.
* ABS malfunction (less common for solely rear lockup): While the Anti-lock Braking System (ABS) is designed to prevent lockup, a malfunction specifically affecting the rear wheels could lead to them locking. However, more often an ABS malfunction will cause overall braking instability.
* Contaminated or worn brake lines (rear): Restricted brake lines can limit the flow of brake fluid to the front brakes, causing the rear brakes to be overused and potentially lock up.
It's important to note that diagnosing the cause requires careful inspection by a qualified mechanic. Simply trying to adjust brakes yourself without understanding the system could worsen the problem.
