What is it doing that leads you to believe that it is on its way out? About a year ago, I lost my clutch and during replacement, the plunger fell out of the slave cylinder and air got in. DC told me I had to buy a whole new unit as it is not serviceable. IIRC, the price they quoted me was about $150. I figured I didn't have anything to lose, so I took the unit apart and bled it (just like brakes). I put it back together and it has been fine for about 20K. And it saved me $150. If you take the entire unit out of the truck and put it on the bench, it will soon become very obvious how to do it. I filled the reservoir, then took a drain pan with fluid in it and put the plunger end in and just kept working the plunger in and out until all the air came up the lines into the reservoir. Takes quite a while but seems to be worth it.