A flexi hose that weeps is usually the olive you reused
I changed a basin tap in June and kept the old flexi because the braid still looked sound. I slid the same olive back up the copper, did the nut up, and opened the isolation. A bead formed at the compression and ran down the pipe. I tightened it. The bead got thinner and did not stop. I isolated again, undid the nut, and the olive was a flattened ring with a bite already in it from the last joint. I had asked a used crush to seal a second time. It would not.
A flexi hose is a short braided lead from the isolation valve to the tap tail. One end is often a nut on a washer. The other is often a compression nut and an olive. The olive is a soft ring. The nut crushes it onto the pipe so the metal bites and seals. That bite is once. A reused olive has already taken a shape. It will not take another. People tape it. PTFE that leaks is usually the wrong way round is for a thread, not for this. Tape on a compression olive stops the bite. You get a leak you cannot pull up.
I isolate first. An isolation valve that will not close is usually the slot. I shut it, I test the tap, and I only then undo the flexi. If I cannot isolate, I stop. I do not start on a live tail.
When I remake the joint I use a new olive. I slide it on square, nut first, then the olive, then I offer the flexi up. I tighten until it is firm, not until the nut screams. I open the isolation and I watch. A weep at the compression is the olive or a scored pipe. A drip from the spout is not this job. A tap that drips is usually the washer in the head. A weep at the tap-end nut is the washer in that nut, a different disc. I still do not reuse a crushed olive on the pipe.
I look at the copper. If the old bite has left a deep groove, a new olive may sit on a scar and still weep. I stop if the pipe is chewed and I cannot cut back to clean tube. I do not re-pipe a bathroom for a scar I can see. I do not hide the scar in tape.
Skip the old olive because it is already on the pipe and you cannot be bothered to cut it off. Skip two olives stacked. Skip PTFE on the olive or under the nut as a substitute. Skip a flexi that kinks when you do the nut up. A kinked braid is a burst later. Give it a bend that is open, not a fold. Skip a lead that is too short so every nut is under strain. Strain opens a joint that would otherwise hold.
The braid can look fine and still be the wrong length or a reused end. I measure with the tap sitting where it will live, not with the flexi in my hand. I buy a new olive with the new lead if the old one came off.
This shows the first time you run the tap after a Saturday change, or as a stain in the cupboard a week later. I now isolate, watch, and put the basin contents back only when the pipe is dry.
I do not need a new basin because the flexi wept. I need a new olive, no tape on it, and a stop if I cannot isolate.
If I later add a buy link here, it will be a paid promo.