I've heard people mention it before. Think it just needs a service, clean and some more high temp grease.
Contact Forge and they may send you a service kit.
 
Ok, does anyone know if the service kit just include high temp grease?
Living in denmark, it might be easier just to go get the grease at a local store. (When I'm told what to get).
 
the service kit has 2 o-rings, new piston and a sachet of red grease :)

The new piston is overkill as mine was barely marked even after 20,000 miles use.
 
the service kit has 3 O rings, 2 for the piston and one for the lid plus some high temp grease and a spare piston. i made the mistake of just regreasing and replacing the piston, it held vac well at the time but latter failed which took me ages to find as i had ruled the DV out as a posible suspect.