* Proportioning Valve Issues: This is a highly likely culprit. The proportioning valve controls the brake pressure balance between the front and rear brakes. If it's malfunctioning, it could send too much pressure to one rear wheel. A sticking or failing proportioning valve is common in older vehicles.
* Brake Line Restriction: A partially clogged or kinked brake line to that specific rear wheel will restrict fluid flow, causing that wheel to lock up more easily under braking.
* Wheel Cylinder Problem (Rear Drum Brakes): If your Sunbird has rear drum brakes, a seized or leaking wheel cylinder on the affected side will cause that wheel to lock up. A recent brake job might not have addressed this if it wasn't visibly apparent.
* Caliper Issues (If Rear Disc Brakes, less likely on a '93 Sunbird): While less likely on a 1993 Sunbird (most likely to have rear drums), if it *does* have rear disc brakes, a sticking caliper piston on that side could cause a lockup.
* Parking Brake Cable: A partially engaged or sticking parking brake cable could be contributing to the issue, especially if the lock-up occurs at low speeds.
* E-Brake Mechanism (If rear drums): Similar to the parking brake cable, there could be an issue within the e-brake mechanism itself in the rear drums.
* Master Cylinder Problem (Less Likely, but Possible): A failing master cylinder could cause inconsistent brake pressure, though this would usually affect multiple brakes.
* Brake Shoes/Pads (Drum or Disc): While unlikely to be the *sole* cause of one wheel locking if the brake job was done correctly, uneven wear or a damaged shoe/pad could contribute, especially if combined with another issue.
Troubleshooting Steps:
1. Inspect the Brake Lines: Visually check the brake lines leading to the affected rear wheel for kinks, rust, or damage.
2. Check the Parking Brake: Make absolutely sure the parking brake is completely disengaged.
3. Inspect the Wheel Cylinder (If Rear Drum Brakes): Look for leaks or signs of seizing. This often requires removing the drum.
4. Check the Caliper (If Rear Disc Brakes): Inspect for sticking pistons.
5. Test the Proportioning Valve (Difficult without specialized tools): This is best left to a mechanic.
6. Have a Mechanic Inspect the System: Given the complexity of brake systems, having a qualified mechanic diagnose the problem is highly recommended. They have the tools and expertise to pinpoint the exact cause.
Ignoring a locking brake is dangerous. Don't drive the car until the problem is fixed. The issue could lead to a loss of control and an accident.
