I now need auto water change programming. When draining, the return pump need not stop. When filling water, need to open the RO water valve and stop the return pump. When the water change is over, the return pump will still stop.until I manually maintenance M4, the return pump will resume. The problem is to lock the return pump after the water change is over, until the M4 maintenance is started manually, so that the return pump is released. This is the core issue. Others are relatively easy to be solved. Thanks!