I recently changed the oil in my WP PDS shock off an '02 KTM 250 EXC. I heard elsewhere you need a special tool to properly bleed the air out of the shock.
I heard Langston Racing sells a bleed tool, but it costs $2200.
You need a vaccum pump similiar to the ones used in HVAC and refridgerant systems. There is no way to get all of the air out of the shock any other way. Factory Connection and MX-Tech come to mind as places that will do this for you. If you replace the piston style PDS w/ a bladder kit, you can do oil changes like the other brands. I bought one on Ebay, it is made by R&D Racing in Cali. I will install it over the winter,,,,,