I have a couple of older Skymaster jets that had the same problem. Over time, fluid will start leaking and no air will remain inside.

I solved the problem by using high viscosity silicone like the one used for RC Cars. http://www.ofna.com/fluids.php

The one that originally comes from the factory is way to thin and and with a higher viscosity you won't have a problem again. Currently I'm using a 7.000 weight.

You can also try to use something like the Dow Corning 55 Valve Lube to lubricate the o'rings and keep them in good shape.