Anti-Siphon valves are an integral part of the marine fuel system. There main function is safety related. They prevent a ruptured or leaking fuel line from filling your bilge with explosive fuel.
How an anti-siphon valve works is actually quite simple. Attachment 427The valve which is really a fitting has an orifice through which fuel passes, on the downstream side of that orifice is a small steel ball. The steel ball is held against that orifice with a spring. If their is no suction from a fuel pump on the downstream side, the spring presses the steel ball against the orifice effectively blocking the fuel flow. Once the fuel pump develops sufficient pressure the steel ball compresses the spring and unseats allowing fuel to flow.
Debris or corrosion due to water in the fuel are the two most common culprits that cause these Attachment 429valves to malfunction. If you suspect you have a malfunctioning anti-siphon valve you can remove it and install a threaded nipple into the fuel line to see if the problem goes away. If this resolves the problem you can try cleaning the valve with carburetor cleaner or by blowing air through it with an air hose. If it is clean you should be able to blow air through it in one direction only. If the valve is bad don't leave the fuel line unprotected without an anti-siphon valve, replace the faulty one with a new one immediately.