* Restricted Brake Line: A partially clogged or kinked brake line leading to that specific wheel is a prime suspect. The hard pedal indicates insufficient fluid flow throughout the system, but the blockage prevents enough pressure from reaching that wheel to engage properly, thus causing the one wheel to lock up.
* Faulty Caliper Piston (Despite Replacement): While you replaced the calipers, there's a chance the new caliper's piston is sticking or damaged. It's less likely than a line issue, but possible. A seized piston in a new caliper is rarer but can happen.
* Internal Leak in Caliper (New or Old): Even a new caliper could have an internal leak, allowing pressure to bleed off. This could present as a hard pedal (because fluid is going somewhere other than the brakes) and potentially cause a wheel to lock (inconsistently or if there's a slight restriction elsewhere).
* Caliper Mounting Issue: The caliper might not be correctly mounted, restricting its movement or creating uneven pressure.
* Wheel Cylinder (if Drum Brakes): If you have rear drum brakes, a seized or damaged wheel cylinder on that particular wheel could be the culprit. A hard pedal is indicative of a restriction.
* Proportioning Valve Issue (Less Likely): While less common, a faulty proportioning valve (which adjusts brake pressure between front and rear brakes) could theoretically cause this problem. This is less likely given you've replaced so many components.
Troubleshooting Steps:
1. Brake Fluid Level: Check the brake fluid reservoir. If it's low, there's likely a leak somewhere in the system. A persistently hard pedal can *look* like a lack of fluid, but a leak is usually somewhere apparent.
2. Visual Inspection of Brake Lines: Carefully examine the brake lines leading to the affected wheel for kinks, damage, or rust.
3. Bleed the Brakes: Thoroughly bleed the brakes, starting with the wheel that is locking up. This will help remove any air bubbles and ensure proper fluid flow. A poorly bled system can act strangely. If the problem persists *after* bleeding, it's unlikely to be air.
4. Inspect the Caliper (again): Even if it's new, double-check the caliper piston movement. Try to gently push it in (with the caliper off the car, and taking care not to damage the seals). If it doesn't move freely, it's defective.
5. Pressure Test (if possible): A pressure test of the brake system can pinpoint leaks or restrictions. This requires specialized tools.
6. Wheel Cylinder Inspection (if applicable): Inspect the wheel cylinder on the rear drum brakes if equipped.
If you're not comfortable performing these checks yourself, take it to a qualified mechanic. This is a safety-critical system, and a misdiagnosed issue could lead to dangerous brake failure.
