Last week I discovered that my preload adjuster was not working and after checking with the guys on the board I figured the oil was gone. This appears to be a very common problem, and can be attributed to slow leaks over time due to O-ring failure. If this is what has happened, you shock will look something like this, without no (or very little) preload active.
This is really pretty easy job to do once you figure out how the preload adjuster works. Here are the step-by-step instructions on how to disassemble, fill it up and reassemble. In my case I took the shock off the bike for a different reason, but you can do it by just dismounting the adjuster unit and working on it while the shock is in place.
1. Turn the adjuster as far counter clockwise as you can (decrease preload). This will save you a bit of effort later.
2. Take off the screw and the washer off the black plastic adjuster knob and remove the knob.
3. Now you see the hex nut which sits on the brass shaft, and there are two (some people have four) little balls underneath it.
4. Take out the balls and springs and do your best not to lose them. They are really tiny. Now comes the fun part of taking off the brass cylinder cover. This is the only part of this work where you may need a specialized tool. I went to a local hardware store and got this 10 dollar tool for removal of grinder discs. It worked perfectly. You may need to mount the adjuster back on the bike in order to crack the cylinder cover open. Mine came apart easily. Turn counterclockwise to open.
5. Here is what you should see when you open the cylinder. Note that one of my washers is stuck to the bottom side of the cylinder cover.
6. Now grab that brass shaft and wiggle the whole piston assembly out of the cylinder. You should end up with this
7. Note that the brass bolt with the hex top (the one you pulled on to get the thing out) screws in and out of the piston body. Inspect the O-ring on the piston - this is the most important part of the assembly. If it does not look healthy, replace it. You should probably replace it anyway. As you see, I had zero oil in the bottom of my cylinder. The oil should be at the bottom of the cylinder, so piston can press on it.
Fill up the cylinder up to the 45-degree point, ie right under the point where the vertical valley (the one cut into the cylinder wall) ends.
8. Now grab your little assembly consisting of the piston and the brass bolt thingy. Turn the bolt counterclockwise until it is screwed into the piston all the way. Then back it out a turn or two. This state of the assembly will represent minimal preload. Put the piston assembly into the cylinder body, making sure that the pin on the side of the piston fits in the sidewall valley (it wont fit otherwise). Press on the assembly a bit. O-ring should ensure the sealing and you should see no oil coming up from underneath the piston. This is what you should see
9. Now put the washer-bearing-washer on the brass bolt... you should see this
10. Put the brass cover on and turn it clockwise with your sexy tool (that sounds so wrong) until it is screwed on all the way. Obviously the assembly will resist a bit since you are putting pressure on the oil a bit. Et voila:
11. Now put back the little springs and the balls on top of them. Put back the metal nut, then plastic plack knob goes on it, and the washer and the little screw that keep it together. You are done!
12. Grab that plastic knob (yeah, sounds wrong again) and crank it clockwise as far as you can. As you are cranking it, the top part of the spring should start to move and you should see the sleeve starting to come out of the top part of the shock assembly, showing some lines. When you get to the far end of the preload (maximum, turning clockwise) you should see something like this... this is my case, yours might be a bit less if you put less oil than me.
If you are interested to learn about my artistic abilities and how this thing operates, here is a diagram and an attempt at the explanation: