fuel injector pump blues

hi folks - bit of a head scratcher here. i've discovered a leak on my 300tdi's injection pump. the leak is coming from what appears to be a vent on the pump, located between the fuel return pipe and the vaccum supply to the top of the pump. it's pretty easy to spot, its got a rubber cover on the end with a breathing tube. when this part is removed, there is a whole drilled through it within which sits some kind of webbing material- i'd attach a photo, but being a ludditte - i havnae got a digital camera! any way, has anyone come across this fault, is it possible to repair 'on the drive' or will it need to go to a fuel injection specialist???