So yesterday we discovered a leak under the sink, it was coming from the supply line into the valve, (suppose the seal broke after several years of use, there was no instigating incident to cause this), well we have tried 2 new hoses, and a new valve and we are getting the same leak! We cannot get the seal to work properly, we've tried tape, no good, it is coming right at the joint where the flex house meets the hardware as well as just around the nut. Anyone have any ideas or tricks they may help us figure this out???enter image description here

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    Is this shutoff valve indeed a free hanging thing between two flex lines? If so this seems rather odd to me. All sink supplies that I have seen use a shut off valve that is joined to a fixed pipe that comes out of the wall behind the cabinet or up through the floor from below the cabinet. I would guess that 95% of the shutoff valves from the store are a type is designed to connect into a fixed line on one side and the flex line from the other side. There is a very good chance that the valve that you have is not designed for the types of seals used on flex lines from both sides. – Michael Karas Aug 30 '15 at 16:58
  • Typically on the supply side, it should be not able to flex. @MichaelKaras is 100% correct. Also, how are you using teflon tape? It should not be more than 1 or 2 circuits around the male end. It is to provide a lower friction for the nut to more snuggly tighten down. – BrownRedHawk Aug 30 '15 at 17:25
  • it actually may be that the valve is not necessary at all, the homeowner before us did the plumbing and the flex lines are a little short, the replacement I bought last night could be long enough to go directly from the wall to the sink, let me see if this works – jfw Aug 30 '15 at 17:46
  • These types of connections shouldn't use PTFE tape. They seal by using a rubber washer. Also, there needs to be a valve under the sink. Do you currently have two in the water line? Then it's OK to remove one, so you still have one in the end. – Pigrew Aug 30 '15 at 17:55
  • Thanks everyone for the responses! We cut out the straight free hanging valve, it was being used to connect the two flex lines. We used the longer line to connect directly from the wall to the sink (there is a shut off valve on the wall). This is what happens when you try to undo someone else's crappy plumbing hacks. :-) – jfw Aug 30 '15 at 19:09

It looks like someone used a straight 5/8 compression x 3/8 compression stop valve as an in-line valve. This picture shows what that valve looked like new:

enter image description here

Your valve has the wrong threads for that flex line (larger end). For a quick fix, you could use a 1/2" MIP x 3/8 compression adapter:

enter image description here

If you must have a valve there, you could use a small ball valve and a couple adapters to make the same connection. Be aware that you may have damaged the threads on the flex line due to cross-threading it. If it was me, I would use a single long flex line, rather than connecting two.

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