I would like to add new supply valves under a bathroom sink, so I can shut off the water to the sink without turning off the main water supply valve. I want to eventually replace the sink and would like to take my time! The problem is, the current elbow joint is screwed in to the copper and I can't get it off. (Of course I'm afraid to put too much pressure on it because I don't want to break anything. I have attached a picture of the existing set up. Any ideas!

I have the same fittings on my lavatory. They are 1/2 thread by 1/4" comprssion elbos. You will need two 1/2" thread X 1/4 " compression angle stops, (see image). Shutthe water off to the house and undo the supplies from the elbos. Put a wrench on the elbows and turn them out counterclockwise. Teflon Tape the threads and install the angle stops. You should be able to connect the old supplies but if you have a problem click on back and we will help. Good luck, Tom

You can use a flarenut wrench to help remove the compression nut. It won't round off the edges. It's basically a box wrench with a slot cut out in it.

Thanks for the information. The biggest problem is that it won't budge when I thy to turn it with a wrench. I was assuming I should turn it counter clockwise, but it was good to have that confirmed. I will see if I can use the flarenut wrench on it.

For removing the valve, a soft jaw pliers on the pipe.

For the compression nut removal, put a wrench on the angle fitting as well.

I would not plan to reuse the existing supply tub between the sink and the elbow so I'd just cut it and then remove the elbow. KISS soft plyers on the pipe from the wall and Tom's angled stop cock and you are in business. A nice new stainless stell braided supply line will have the correct fittings on each end if its indicated for sink supply, about $3.50 and the angeled stops are about $5 each. Buy 1/4 turn stops if you can find them.