Daz sorry mate but I've been repairing machines for many years now and the normal cause is through neglect.ie (not keeping the machine well maintained I have regular customers and through neglect have the same problem)
After a damn good clean out they work OK (on the air line that is)
I agree it may be the spring but take the easy way first?
As pleasant says without knowing what it is and actually seeing the machine It's hard to rectify.
salvager, October 2011