If you have any amount of plastic tubing, like the 1/4 plastic tube used to feed the water fountain on the sink, or the icemaker and water dispenser on the refrigerator, you should get one of these coupling joints, part number PP040WD, PL-3000, SKU 707102. It’ll save you a lot of headache when the plastic tube leaks.
We had a leak under the sink, where the water filter is. This is a really old installation going back around 30 years, and it’s held up, but the plastic pipe keeps breaking every decade.
Here’s the connection to the water supply. Yeah, it’s piercing steel tubing.

So, first, I shut off the water supply, so the leak wouldn’t be so bad.
It wasn’t enough, so I opened up the faucet to relieve pressure. Water dribbled out, and the leak abated.
I tried to fix this leak with Gorilla Tape. The stuff is amazing, but it didn’t seal up this leak.

So, what I did, was cut the leak out:

I just cut it out with scissors. Just try to cut the ends square. My cuts weren’t great, but they worked.
Then I mended the break with the coupling.

There you go: a quick, temporary fix.
Part Two: the more permanent fix
I then went and bought several pieces of plumbing online. John Guest has a valve that fits in between a standard 3.8″ compression supply line, and presents a 1/4″OD push-on fitting.
Since this sink’s water supply doesn’t use the contemporary 3/8″ connections, but the old 1/2 MIP connection, I got two 1/2″ FIP – 3/8″ FIP supply hoses, and a (male-to-male) 3/8″ MIP union coupling.
I hope this all works, and we’ll have a new supply line, with a 1/4″ push-on connection in the middle.
Then I’ll be rid of this three-decade old plastic tubing, which will probably leak again.
Then again, the replacement tube will probably leak, as well.
That’s why you need one of those 1/4″ to 1/4″ union couplings.