replacing faucet isolation valves

I'm replacing the faucets and associated isolation valves in my bathroom vanity and have a general question. The valves are threaded connections with brass ferrels and nuts. I cant figure out how to get the existing ferrels and nuts off the copper pipe without cutting the ferrel and I'm afraid of marring the pipe. Is there a risk of these ferrels leaking if I hook them up to the new valves? They dont currently leak but the valves are getting old and corroded.
thanks, dave
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Dax wrote:

Leave the old ferrules on. The tubing has been distorted (crushed) under the ferrule.
Apply a little sealant to the ferrule and connect to the new stop valve. Don't try to overtighten; just real snug should do it.
Jim
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If you are using a new faucet and new supply valves, why would you use the old pipe connections. Get rid of them and get flexible (metal braid) connection tubes at the hardware store for a few bucks a piece. They come in 1/2", 3/8" and 1/4" connections that will mate with the compression fitting (nut and ferrule) on one side and the sink threads on the other. (there are a bunch of styles and lengths so choose carefully)
If you insist on using the rigid supply tube (chrome plated copper or solid copper) try reassembling as is at first (as the other poster suggested) otherwise, you need to start fresh with new tube, nut and ferrule as the connection requires the deformation of the copper metal which is not reversable.
Cost should not be an issue because however you go, its less than $15 in parts plus your labor.
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