The hatsan 125 and 135 gas pistons have a fill port. you would need a Hatsan fill probe, and a PCP compressor or hand pump. The fill port is on the starboard side, near the rear of the piston. you need to remove ther stock to access it. There is also a bleed valve on the port side.


Check these vids out for visuals.

https://www.youtube.com/watch?v=-WUrYNfZ_n0

https://www.youtube.com/watch?v=EZb7Es9Ypck&t=621s