Sorry Julien in advance! Just purchased a very ncely part built two and a half nch gauge Flying Scotsman. I was told it had worked on compressed air, but when an airline was connected no go. But when I disconnected the air and applied to one "side" of the motion off it went. I have just stripped the "non working cylinder down and it seemed OK, but after re-assembly the problem is still there. To clarify the "non working" side will not work when air is just piped in to that side. Any "positive" suggestion very welcome!