Leak under bathroom sink

I have a leak where the cold water supply line connects to the faucet. I disconnected the line from the faucet and tried replacing the slip-joint washer, but when I reconnected the line to the faucet, the leak actually got worse. It seems like the new washer is not creating a good seal. Does anyone have any recommendations on how I can stop this leak? Do I need to apply some kind of threading compound before slipping on the washer?

Thanks

leak

What kind of supply do you have?
What size is it?
Describe it.

supply line

The line is about 20 years old. It's a stiff metal hose that has ridges. It was connected to the faucet with a brass bolt, and when I loosened the bolt there was a rubber washer around the hose at the connection site. I'm not sure about the size.

leak

Quite often that type of connection leaks because the tubing has a crack between two of the corrugations.

If I'm reading you right, it sounds like a banjo fitting. Didn't know they used them on water connections. If you don't clean off the surface (there is likely some old rubber particles stuck there), it may not seal right. If you overtighten it, it can distort, too. Does the metal washer look like it is cupped? You might try just flipping that and see if it works better. My unprofessional opinion.

leak

Uh oh. Well, I definitely see the water squirting out from underneath the washer, but I guess there could be a crack in that spot.

Someone suggested I replace the line with a new flexible one, but I want to avoid shutting off my main water supply, if I can. I live in a condo, so it's a little difficult to do that. Looking at the shut off valve that's under the sink, it looks like I would have to remove the knob in order to disconnect the old hose. Would the valve remain off if I remove this?

The knob shouldn't have to be removed, but even if you do, the water should stay off. Shut that valve off, see if it actually still works, and if it does, replace the hose.