My brother has a GML S400 too.
Not shot it for 4yrs but our dad wanted to have a plonk yesterday. The S400 still had air in it, was on 150BAR, and first shot was spot on, no re-zeroing.
These were well regarded custom tuned actions back in the day. Nothing wrong with the old type filler. Probably just a seal or two needs doing on yours.
They are quite straightforward to service so long as you're absolutely certain there's no air inside before you start disassembling!