Gary,
Tarakihi has a "garden hose" type Y connector in the water supply from our single "Barrel" water tank. The connector has in-line valves in each of the branches. If we remember to keep the valves closed whilst sailing hard no water is ejected from the taps in the heads compartment or galley. If we don't it gets rather wet!! Our "tank" is also vented using a domestic push-fit valve on a flexible 15mm flexible braided tap connector hose. This small air bleed might also help to stop syphoning but is probably to stop "flow-back" when the tank is filled from the deck-mounted filler fitting. I don't know if these arrangement were fitted by Parker's as we are the second owners but they seem to work well.
Chris Cobb
235/48 "Tarakihi"