Might be a problem of some sort with the auto-adjuster, ABS, proportioning valve or the parking brake mechanism, but my money would be on the brake hose connecting the brake cylinder to the brake lines. The inner lining of these hoses sometimes detaches when the hoses get elderly and blocks the hose. It doesn’t prevent braking because the pressures when the brakes are applied are pretty high. But it can prevent/delay the release of the brakes.