Supply Line Connection to Shut-Off Valve

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Dandyman123

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I installed a new bathroom sink faucet and the existing Hot supply line connected fine, but I could not get the Cold supply line to screw in to the faucet end. So I bought a new supply line and removed the old one (both are 1/2 inch lines). I connected the one end to the faucet, but can not get the other end to attach to the shut-off valve.

I have not replaced supply lines before so I'm not sure what the shut-off valve connection should look like, but as you see in my first picture, there is a black rubber gasket or something on the shut off valve connection. I don't know if that belongs there, or if it came out of the old supply line. I want to make sure before I try to remove it. The 2nd picture are the two ends of the supply line that I removed. The end on the left side of the photo is the end that was connected to the shut-off valve and it does not have a black rubber part in it like the end on the right which was the end connected to the old faucet.
 

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This is what you need under a sink. Your supply line should be 1/2" at the faucet and 3/8th at the valve.
 
How do you know it should be 3/8? The tag was still on the old supply line and had two measurements on it, 1/2 inch and 1/2 inch.
Also, does that black thing in my 1st photo need to come off or does it belong there?
 
They also come in a 1/2" leading to the supply line. I prefer 3/8" but that is just me.
 
I’m confused, but of course I’m new to this. I don’t get to choose whether I use 1/2 or 3/8, right? I need whatever size fits the existing shut-off valve. Here is a pic of the tag on the old supply line. Does this tag mean both ends are 1/2, and therefore the replacement supply line should also be 1/2 at both ends?

And back to my original question, does that black rubber piece belong on the end of the shut-off valve outlet or did that come off of the old supply line?
 

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That black rubber piece belongs on the supply line. It's shot. You need to buy a new one. Is your intention just to leave everything as it is, or are you buying any new parts?
 
I’m just trying to hook up the new faucet I installed. The only other new part I bought was the new supply line for the Cold faucet because I could not get the old one to attach to the new faucet. The more parts I replace, the more I’m getting over my head.

So, I can remove that black rubber piece from the shut-off valve and then try to connect the new supply line? I just wanted to make sure the black thing didn’t belong there before I remove something I shouldn’t have.
 
Remove the black rubber, the new supply line will have one inside.
Clean off any bits stuck onto the supply valve.
The flat round face of the valve needs to be clean, for a good seal.

Get a new flex supply line marked “faucet” with 1/2 inch ips or iron pipe or fip on both ends of it.

Long enough to reach both connections.
 
His supply valve looks like it might be from the 60’s.
1/2 inch iron pipe supply was not uncommon then.
Yes, 3/8 compression is very common now.
 

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