Walther LG 90 reseal/piston

[ultrafreak]

All,

I have an LG 90 that's been shooting under-power. I acquired new breach seals and the Dichtung seal (whatever that's called; the air chamber seal) and replaced the old seals. So far, so good. But even after replacing these, I still am seeing low power shots. If I pump the rifle and let it sit for a couple hours it maintains some pressure but it's clearly not holding air. In fact, when I pump the rifle I hear a pfffft on the compression stroke as a bit of air is released.

So... I'm afraid I need to replace the piston. Any tips on how to do so? I know in the FWB's cylinder lube is either strongly discouraged or supposed to be very sparse. Do Walther SSP's require any lube in the piston?

thanks in advance,
Jeff