Not too long ago, I bought a .177 Benjamin M-Rod synthetic to take over indoor practice duties from my venerable RWS Model 75 10 meter "double springer" (which I cherish).... But I'm afraid the seals will go out on the old gun, and I dont know if anyone's still around who knows how to reseal them. I hear it's a bear to do....

But I also just got tired of my M-Rod leaking all over the place and I'm tired of forever pumping it up, so I yanked the RWS Model 75 out of the safe, slapped on a UTG, and I'm happily banging away at a biathlon paddle target on my home range. I'll shoot this old gun until it dies, and then look for a used SSP 10 meter to replace it.

Those giss contra guns are difficullt to rebuild, but not terrible. The main thing with them is timing the rack gear. The factory has this covered with the proper shims already inserted into the dead piston end cap. Take care not to misplace any of these and you should have no issues. Sourcing parts will be a bigger issue, but chambers in the UK should have them.
But i'd shoot it as is, till the need arises.

http://www.airgunartisans.com/AACrookedBarn.htm

It's been several years. Did a fantastic job.