mike, the check valve is located on the side of the room manifold. its a 9/16 brass fitting with a spring and check inside.
be careful when you remove it. dont lose the spring. the part number is RAP10021. its less than 10 dollars, and includes several different checks, and a new cap. it doesnt include the spring.
fossil, the cylinder leak would make the slide move after its extended or retracted, but when you are running the slide, it should equalize and move in or out evenly. the extend solenoid valve is mounted higher on the manifold than the retract solenoid. the harness is also marked