If a piston filler empties itself shortly after being filled, so that the cap stands full of ink and the barrel is empty, the cause is almost always the piston seal: over the decades the sealing ring, or piston washer, has hardened, shrunk or cracked and holds neither ink nor vacuum. Less often a loosened feed, a poorly sealing section or hairline cracks in the barrel are to blame. A pen in this state should not be written with, because ink creeps into the cap and the threads and attacks the metal parts and engravings there.
The remedy is to replace the seal and overhaul the piston mechanism; that is routine work for a specialist repairer and, as a rule, nowhere near a write-off. For OMAS pens an independent workshop is regarded as the more dependable route, and among collectors Tom Westerich, who specialises in the marque, has made a name for himself in this line of work. Collectors report that the factory service has been of little help since the parent house was taken over by the LVMH group, and for older models it is seldom the first choice in any case.