Replacing a leaking cutoff valve for a toilet or faucet is pretty simple, expect for when the old ferrule/compression ring get stuck on the water pipe. This happened to me the other day, and I did not have a puller tool available, but an idea popped into my head that I might be able to use the old valve to pull off the frozen ferrule. I gave my idea a try, and it worked. Here is what I did:
It ends up 1/4″ washers fit perfectly inside the old valve. I put enough in so that the end of the pipe will push against washer and the nut will pull on the stuck ferrule.
Tighten the nut until the ferrule hits the valve, take the valve back off, add another washer, and repeat until the compression ring is almost off the end of the pipe.
Now a gentle twist and pull should take the ferrule off. Clean up the pipe with some sandpaper, and you should be ready to put on the new valve. Not a bad trick for a DIY plumber to know about.