Couldn't figure it out. Kept losing water. Started with 8 oz every couple of days, then quart per day. Tried everything. Searched and tightened. Figured it must be a head gasket. Laid awake thinking and worrying about the $$$$. Ended up being a separation in the seam of plastic expansion tank. You could only see it under pressure. Tested tank with compressor pressure and found it. $25 replacement. Problem solved. Just wanted to share to save someone else the pain and misery.