The invention comprises an adjustable valve system, apparatus and method using an adjustable drum assembly that can be used in conjunction with a blower or pump to drive air into and out of the drum assembly, wherein the adjustable valve system is useful in connection with operating wave generator caissons for wave pools. The present system preferably comprises an inner drum coaxially aligned and rotatably positioned within an outer drum, wherein side openings are provided on the inner and outer drums which can be aligned together to allow air to flow into and/or out of the drum assembly. That way, by adjusting the position of the inner drum relative to the outer drum, the valve system can enable the air to flow either into or out of the drum assembly and therefore the associated caisson. The system can be used to introduce positive or negative air pressure in the caisson and create waves in a wave pool.