* Proportioning Valve Issues: The proportioning valve controls the braking force between the front and rear brakes. If it's malfunctioning, it might send too much pressure to the rear brakes, causing them to lock up before the fronts. This is a very common cause of rear brake lockup. It could be sticking, failing internally, or simply worn out.
* Rear Brake Caliper Problems: Seized or sticking rear calipers will cause the brakes to drag and potentially lock up. This is often due to corrosion, damaged seals, or a faulty caliper piston.
* Brake Lines: A restricted or kinked brake line to the rear brakes can increase pressure and lead to lockup.
* Parking Brake: A partially engaged parking brake can contribute significantly to rear wheel lockup, especially at lower speeds.
* Wheel Cylinders (Drum Brakes): If the Achieva has rear drum brakes, the wheel cylinders could be leaking internally or sticking, causing the brakes to drag. Leaking wheel cylinders will also reduce braking effectiveness.
* Brake Shoes/Pads: Extremely worn rear brake shoes (drum brakes) or pads (disc brakes) could, in theory, contribute, but this is less likely to cause a *sudden* lockup. Instead, it usually leads to gradual reduced braking efficiency.
* Master Cylinder: While less likely to cause *only* the rear brakes to lock up, a failing master cylinder could lead to erratic pressure distribution, potentially affecting the rear brakes disproportionately.
* Brake Booster: A faulty brake booster could also contribute to uneven braking force.
Diagnosing the problem: It's crucial to have a mechanic diagnose the issue. They'll likely inspect the brake system components mentioned above, checking for:
* Fluid leaks: Look for fluid leaks around the calipers, wheel cylinders, and brake lines.
* Caliper movement: They'll check for free movement of the calipers.
* Parking brake operation: They'll check the parking brake to rule out partial engagement.
* Brake line condition: They'll inspect the brake lines for kinks, restrictions, or damage.
* Proportioning valve function: This might require specialized testing equipment.
Ignoring this problem is extremely dangerous. Locked-up rear brakes can lead to loss of control and accidents. Get it checked immediately.
