A fine hypodermic syringe is the tool for getting the gas back it (obviously with the attachment to connect it to a regulated nitrogen bottle). Alternatively, its fairly easy to buy a high pressure schrader (normal tyre type) valve and modify the shock to accept it.
Same syringe can be used to empty the gas out, or you can simply slowly unscrew the top of the diaphragm cover. Most of the time, there's no gas left in them anyhow.
As the parts book suggests, its all pretty straight foward. They're harder to bleed than most shocks, but a little patience will prevent it being a drama. Also, accept that you'll make a mess when it goes back together.
Three likely pitfalls:
1. Rust on the shaft under the felt top seal.
2. Torn/split gas-oil seperator diaphragm. Have been told that they're very hard to find new.
3. You local bike shop won't be able to put enough pressure into it. The gas pressures were REALLY high in these old shocks, as the gas was intended to provide some of the spring effect. Walter/YSS can supply stiffer springs, allowing you to drop the gas pressure down to modern levels.
I am looking at converting my YZ125X to a modern valve piston, but it looks like being a BIG job.
