It sounds to me that the air check is below the grid plate. I've seen this happen many times. The air check should be centered of the grid plate. If it's not the bottom, the fill time will not allow enough water to come above the grid. Measure the distant from the bottom of the brine tank to the bottom of the elbow of the brine line. Then measure the from the bottom of the air check to the bottom of the elbow. Bet they are real close. Should be about 4" difference.