Not many that I have heard of rebuild the unloader, kind of like guns, they usually just replace them. If you want to try and rebuild the unloader, the great vendors like Russ and Paul and Jerry can get you the kits but you are waiting for the kit to get there and then taking it off, apart and hoping the kit will do the job where if you just order the unloader (get a spare also so you have one on hand for later on) then just change it out.
Before you remove the unloader, take a good look at it and take some pics so you get the new one correctly installed but also look to see how you can install the new one easier and use another jumper hose with swivel ends or other parts so the next time will not take as long. First time I replaced the unloader I had to replace a couple hoses as their ends were very rusty and the hoses inside did not look all that good so I did not think that they would last too long so they were replaced.
If the unloader is in a bad spot, get one of those brass unloader blocks and move it to a place where it is easier to work on or replace. Send the vendors a picture and they will help you and get you what you need.