If the water stops when you stop the timer, the water valve is fine. You have a loose or broken air line tube running from the back of the tub up to the pressure switch.
Basically you need to remove the front panel and the main top so you can find the hose problem. Sometimes an extremely off-balance load can dislodge the tubing...sometimes mice will chew thru it. It may have just popped loose, but either way, the pressure switch on the control panel is not recognizing a full tub.