I have a Steyr 91 match co2 rifle and the co2 cylinder is only allowing a small amount if any co2 out. I have pushed in the centre pin and I can hear a gentle and irregular flow of co2 when it should be a powerful whoosh/hiss. I assume there is a valve that needs attention in the cylinder and wondered if this is an easy diy job, if so any ideas on how to tackle it? Thanks for any input.