I have a problem wit one of my beer engines. In the middle of dispensing from a Firkin Cask the beer engine has stopped bringing the beer up to the tap but is producing just air. I have checked the hop filter (cleaned regularly once a week when the lines are cleaned) and that is clear. I have put the spear into clean water but it still will not draw any water so a vacuum in the cask is not the problem. There is no suck back when the handle is pushed into the upright position. Can anyone help please??