I found the unit when mounted on cylinders station thedidnot switch over to the full cylinder from the empty one. On ivestigation and replacing the cylinder connection hose with one from the old changeover unit it works as expected. One further inspection of te hose it was found that the non-return valve was jammed closed by a dirty-black greasey "gunge". I was able to take the coupling apart and clean the valve of dirt and further into the hose, I am reluctant use the hose incase it is not totally clean.I am also concerned that dirt may have blowh into unit and cause a problem in future. I am disappionted at your poor quality control .