There may be dirt in the lines or the calipers. When you press the pedal it forces fluid into all 4 brake calipers. If a line has dirt in it, it might get stuck in one of the orifices, causing them to stay in the locked position. Over time, the pressure in the caliper leaks back out and they release. Have a mechanic do a brake fluid flush, preferable a "reverse" flush to push everything up and out through the brake fluid reservoir. Total cost should be about $30 for the fluid and 1 hour of shop time.
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There may be dirt in the lines or the calipers. When you press the pedal it forces fluid into all 4 brake calipers. If a line has dirt in it, it might get stuck in one of the orifices, causing them to stay in the locked position. Over time, the pressure in the caliper leaks back out and they release.
Have a mechanic do a brake fluid flush, preferable a "reverse" flush to push everything up and out through the brake fluid reservoir. Total cost should be about $30 for the fluid and 1 hour of shop time.