Leaks from the back of an M17 around the regulator may be caused by a few things. To help determine the location of the leak you can spray a small amount of marker oil or soapy water around the outside of the regulator and ASA. Look for the air leak to causes air bubbles.
Leaking from the stock arm screw or around the tank and ASA threads is caused by the tanks regulator o-ring. Replace it with a standard 15-90 polyurethane tank o-ring.
Leaking from the base of the reg cover or the velocity screw is caused by the regulator o-ring on the piston head.
Leaking from the base of the pressure relief valve(PRV) is caused by the PRV o-ring.
Leaking from the top of the PRV is caused by the output pressure of the regulator being too high. Decrease the pressure by turning the velocity screw out in small increments until the leak stops. If the regulator is still leaking from the PRV it may need to be rebuilt.