Rear shock "dead zone"???

This is a new one to me. I just bought a wr400 and the rear seems to have developed a spot in the travel with no damping. It's not making any weird noises, but for instance, on compression, it travels freely and then hits damping. Then, on rebound, same thing. Figuring I had to take it off and mess with it anyway, I pressed the schrader valve and found there was no pressure at all. It wasn't like this the day before, so I am hoping someone can shed some light on what's going on here. Does it just need to be charged? Can I test this theory by pumping it up with air? Not that I could approach the proper pressure. Anyway, any info is appreciated, thanks to all. ...N

How much pressure should I charge the rear shock with? ...N

Thanks for that. I checked it for leaks and pits/scratches/blueing and all appears well. Since it's new to me, I was hoping to ride it a bit this weekend, but I guess I might as well just tear it down and get the forks and shock tuned for my fat-boy specs asap instead. Money well-spent I know but $650 is a hit. Thanks again for the info. ...N

Just for the record, a 150lb charge of nitrogen fixed this problem. ...N