Sorry nobody responded.
Lets get terms correct first. Your parking brake is actuated by spring pressure inside the brake chambers, and released by air pressure from the yellow handled valve. Your service brakes are actuated by air pressure which is provided by the brake pedal treedle valve. Parking and service brakes are two separate things.
It sounds like your issue is with the parking brake, and it not releasing intermittently, versus the service brakes.
From reading that other thread, you've likely learned that your parking/spring brake release can be disabled through different safety interlocks present on the bus. Those interlocks can be controlled by switches on doors, windows, and hatches.
Whenever I see someone complaining on here about parking brakes not releasing, almost 100% of the time it stems from a handicap door, and someone clipping the wrong wire when removing the lift. Have you done any wire tidying or lift removal recently? If not, has the lift door been jiggled or opened in correlation with the parking brake not releasing?