The AAV is a cylindrical-shaped object with a noticeable lip made from PVC or polyvinyl chloride fitted with rubber valves. The valve is one-way, meaning that it only allows water to flow out and not back in.he AAV is a cylindrical-shaped object with a noticeable lip made from PVC or polyvinyl chloride fitted with rubber valves. The valve is one-way, meaning that it only allows water to flow out and not back in.