Its hydraulic. Likely all the oil has leaked out over time.
It can be topped up easily.
I found a thread earlier this year on the forum showing how to do it. I have searched for about 10 minutes now and got nowhere. I am sure if you persevere on the search engine you will come up trumps.
By the way i did mine which made things better but it didn't solve the route of the problem. That basically the rear shock was knackered.
Thats why I am on the keyboard at this time of the day. Awaiting the long, long return of my shock which seems to have spent the most of the last two months travelling the length and breadth of the UK not attached to the bike but in the back of a delivery van. Being driven by some mad delivery driver who can't find my house.
It is the preload adjuster that is just turning (albeit lumpily) and no adjustment is takign place.
I did the top up thing on the link shown in the summer and it was a little down but hardly anything really.... wonder if, as usual, I shoudl have left alone!
Someone will shout otherwise I'm sure but if I buy a rear shock again it'll be an oem one. Going to try a progressive front spring to go with my progressive rear first. Mine exploded and was all ground out of shape. Just my 2 p. that's not a euphemism btw, it's just vat going up by 2 %