The Honda XR400R's fork seals are essential to a smooth ride (it is recommended to replace the fork seals and fork oil every 15,000 miles). The fork seals keep the fork oil in the fork tubes, but the seals can weaken over time, especially if the motorcycle is exposed to extreme conditions. If the seal weakens or breaks, you will notice oil on your bike's slider; when this happens, it is time to replace the fork seals. The procedure for replacing the fork seals is the same for both forks.
Attach the top triple clamp to the fork tube. Make sure that the fork tube is held securely in place, and then use a wrench to loosen the fork seal cap at the top of the fork tube.
Locate the nut that is located just below the fork seal cap -- use the wrench to loosen and remove this nut. With the nut removed, detach the fork seal cap and set it aside. Pull the fork spring out from the fork tube and remove the end spacer (use a socket wrench) from the bottom of the fork tube.
Hold the fork tube in place with your hand and loosen the top triple clamp. With the clamp loosened, pull the fork tube way from the motorcycle and drain it over the dripping pan to remove all of the oil. Remove the old seal from the fork stanchion and then cover the new seal with fork oil -- slide the new seal over the stanchion and tap the top of it until it seals completely.
Fill the fork tube with fork oil (when you are filling the tube, stop at the halfway point and pump the stanchion to distribute the oil throughout the fork tube). Once you have filled the fork tube, pump the stanchion 10 times and then put the fork spring back into the fork tube and reattach the end spacer.
Reattach the fork seal cap to the top of the fork tube and then reconnect the fork tube onto the bike, tightening the bolt that was removed in Step 2.