How to repair leaky pneumatics?
The parts become leaky due to the seals inside the pistons deteriorating.

Typically, the seal is made with an "o-ring" - a circular rubber band (cross section as well as the shape) - if this is the case in the LEGO pistons, and then it's possible the o-rings are made with similar rubber to the LEGO rubber bands, and then they will deteriorate over time in the same way the rubber bands do.

I've not tried dismantling any of my pistons, so I couldn't say how easy they are to repair - they look like you should be able to unclip the black part and access the piston that way (there's a YouTube clip of someone doing so here, you can just see the piston head too, but I can't confirm the o-ring).

A good car spares or similar type of shop should be able to sell a selection of replacements.

From my time spent diving, there's not much you can do to stop this happening over time, other than keeping an eye on them and checking them regularly for replacement.