* Brake Caliper Piston Seizure (Most Likely): A caliper piston can seize due to corrosion or a lack of lubrication. Heat from braking can exacerbate this, causing the brakes to lock. Cooling down (the 20-minute wait) allowed the piston to slightly release its grip, enough to unlock. This is *highly probable*.
* Brake Line Restriction (Less Likely): A partially blocked brake line could cause pressure to build up excessively in one caliper, leading to locking. The pressure might have equalized after the system cooled. Less likely than a seized caliper but possible.
* Master Cylinder Issue (Less Likely): A problem with the master cylinder, such as a sticking internal component, could theoretically cause uneven pressure distribution, but this is less likely to spontaneously resolve.
* Proportioning Valve Problem (Unlikely): A faulty proportioning valve (which balances front and rear braking force) could contribute but is less likely to explain the self-correction.
* Brake Booster Vacuum Leak (Unlikely): A significant vacuum leak in the brake booster would result in weak or failing brakes, not necessarily locking.
What to do:
1. Don't drive the car until the problem is fixed. This is extremely dangerous.
2. Inspect the front brake calipers visually. Look for signs of corrosion, leakage, or damage. Are the pistons sticking out unevenly?
3. Check the brake fluid level. Low fluid could indicate a leak somewhere in the system.
4. Check brake lines for kinks or damage. Look for any obvious signs of rust or corrosion.
5. If you're not comfortable working on brakes, take the car to a qualified mechanic immediately. Brakes are a critical safety system, and attempting DIY repairs without experience can be hazardous.
Troubleshooting Steps a Mechanic Might Take:
* Bleeding the brakes: This might resolve a trapped air bubble, but it's unlikely to be the sole cause given the spontaneous unlocking.
* Caliper rebuild or replacement: This is the most likely repair.
* Brake line inspection and repair/replacement: They will thoroughly check for restrictions.
* Master cylinder inspection and possible replacement: Less likely, but a possibility.
The fact that the brakes unlocked on their own makes a seized caliper the most likely culprit, but a thorough inspection by a professional is essential for your safety. Driving the car with this problem is extremely risky.
