Another thought:  I recently purchased two of the new, large Coleman brand Butane cylinders which are supposed to work with MSR small burners.  They did work the first time I screwed a MSR burner on them and did just fine.  However I left the burner screwed in because I would use it for the next meal and was in camp.  The second time I tried to light the burner it did not want to light...seemed no gas was coming out.  This happened to me with both cylinders and two different MSR burners.  Worked fine the first time, but with burner left on would not light/pump fuel a few hours later when I wanted to cook something else.  Has anyone else had this happen to them?
The cylinders say "self sealing" so I am wondering if they might seal below the aperture which allows the burner to access the fuel when it is screwed into the canister?  My trip was nearing the end so I unscrewed the burners and brought them and cylinders home.  I have not had a chance to re-try them now to see if unscrewing the burners made a difference.  :-? :-?