The flow control knob works very similar to a hose bib or gate valve as its main function is to either fully allow or restrict the amount of water passing through the valve. The main difference is that the flow control limits the diaphragm's stroke, …

All Hunter Industries Anti-Siphon Valves should be installed above ground and at least 6 to 12 inches above the highest sprinkler to prevent back pressure or drainage. Installations at any height lower than this may result in unsafe backflow condition. …

If water is leaking from the vacuum breaker portion, there could be debris in the anti-siphon poppet.  This would prevent the valve from sealing completely.  The cap can easily be removed by twisting counterclockwise to inspect the internal poppet area. …