I recently purchased a house and discovered that the Pressure Vacuum Breaker was leaking (or it's the pvc directly connected to it). I went under the house and shut the water off to the sprinkler system and found that there is no drain valve between the shut off and the PVB. I am already going to work on the PVB leak, but was curious about the lack of drain between the PVB and the shut off valve. There is about 4' of pipe under the house and around 1' outside the house between the valve and the PVB. Is it necessary to have one? What are the downsides to not having one?