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The article below based on the similar VF1000 model equipment inspired me to replace the damping fluid of my RC17 rear shock. I followed the procedure generally as recorded except:

1. Expel the existing fluid by using a large plastic syringe (60ml?) whose end happened to be exactly the correct size to screw into the hose after the valve was removed. A couple of pumps of air were enough to encourage the fluid to come out quickly into the syringe. I left it overnight to drain the remnants into the measuring beaker - and got 340 ml.
2. To fill I set up the shock vertically with the hose at the top and used the syringe to inject the ATF from its measured amount into the hose - much quicker than waiting for it to drain in under gravity.
3. For both of these pumping operations you need to allow the excess pressure to escape, but this can be controlled by opening the syringe to accept it.
4. I chickened out of removing the hose from the body - but I do not see why this could not be done to further aid the operation. Perhaps the hole would not be the correct size for the syringe end, and resealing may be awkward.

I can't claim any great difference in feel on the bum-ometer, but at least I know it's all clean inside. Don't forget to repressurise the shock after reinstallation.

---SRM---

From http://www.one-ring.net/vfrfaq/general.html#absrep

by: Joe Thorne - Foreword by Christopher Leach

With the shocks in our bikes getting on in years, some folk have begun to experience significant sagging and less than desirable damping characteristics in their shocks. Unfortunately, Honda designed this shock as a 'non-rebuildable' shock. This means that the spring is sealed inside the shock and cannot be replaced. However, it is possible to get a few more miles out of them by improving the damping and increasing the air pressure (preload). The following procedure details replacing the shock oil with heavier fluid for improved dampening characteristics. (Editor)

I have never seen a rear shock leaking, but the service manual details how to change the seal. IMHO, if the seal is leaking, get a new shock. If you want to change the fluid, however, the manual still wants you to replace the seal. Ten years as a flat-rate tech, I sure as hell aren't changing a seal that isn't leaking.

Tools needed:
Whatever you need to pull the shock
Graduated beaker
Funnel small enough to screw on the airline for your shock

So the best and fastest way to replace the fluid is to remove the shock. (See your service manual).

Once you pull the shock out, unscrew the Schrader valve from the hose. (The valve screws on the line at the end, 10mm and 12mm hex heads.)

Hold the shock at a 45 degree angle over the graduated beaker, with the valve and hose facing down. You will have to wiggle it back and forth a little to get all of the fluid out. The reason to drain into the beaker is this: My manual called for 300cc of fluid, I got 340 out.

I replaced the crap that came out, which was smoked ATF, with ATF. In hindsight, I would use 15wt fork oil.

To replace the fluid, take your funnel and seal it to the air fill hose. I nailed the funnel to the workbench, and set the shock on the floor.

However you do it, make sure the funnel is well supported as high as possible. I filled the funnel and walked away. Twelve hours later, I walked into the garage to see the last of the fluid drain out into the shock. I kept a small space heater on the shock to keep the fluid as thin as possible.

(Editor's note: When I did this, I used 340 cc of 10WT fork oil. In retrospect, I too would use 15W for a little stiffer ride. Some listers have reported using 30W with no seal problems.

Also, I used an irrigation syringe to force oil into the open hole in the shock body.

Only took about 30 mins. The author's bike is a VF1000R, editor's bike is an '84 VF1000F Interceptor)