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???
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:
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:
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.