Fix my tap please

Having sorted my taps I notice that There is a leak from the dishwasher pipe.
The leak is coming from between the nut and the backboard. This leads me to believe that however the nut is fixed to the pipe it is faulty.
Is it the same principle as a brake hose? Anyway, what is the best fix?

There shouldn’t be any PTFE tape on the thread of a compression joint. It gums the thread up and stops it tightening properly. Probably what has happened here. Take it apart, remove all the tape, put it back together. You can add a bit jointing compound or (controversial alert) some PTFE just around the olive if it still leaks.

They do work like a bike brake so the thread isn’t doing any of the sealing, it is all done by the olive, hence why it is pointless (and in fact problematic) to put tape on the thread

Caveat: is it definitely this joint that is leaking? What’s behind the board? Could there be a leak on the pipework behind it which is trickling down the pipe somehow?

Assuming it is that joint, it’s a compression joint, so there’s an olive which is being compressed onto the pipe by the nut and which will weep slightly if not tight enough.

OR it will weep slightly if it’s too tight. The joys of compression fittings.

I’d try nipping it up slightly in the first instance: to do so you’ll need to constrain the isolation valve with a pair of grips, then slightly tighten the nut with a wrench. Dry the joint off with something, see if it leaks again.

Same principle. There is a squashable brass olive in there. Get a new one of those (different diameters are available, I’d guess 15mm for that but just check) Refit using the new olive, and some PTFE tape on the thread. You want it tight, but is possible to overtighten and make it leak so not too tight

Beaten to it, but Nick and I seem to disagree on tape. IANAP but I’ve done it on all the ones in my house and it’s been fine

It seems PTFE on threads is a Controversial Topic*

* Think Magic Mary v.s. DHR2 but for plumbers

Boss green jointing paste for potable water. Just smear a little on the wet side of the olive & do it up.

Can’t think why you’d put ptfe tape on a compression coupling thread. It doesn’t leak out of the thread bit.

I’m with Hot Fiat.  I fit hundreds of compression fittings a year and my method is a smear of potable jointing compound on the fitting side of the olive, this helps lubricate the thread a little as well, so you don’t need to over tighten.  PTFE tape has no place on the threads, it’s doing nothing and is a sign the ‘plumber’ didn’t know what he was doing IMO.

To fix your problem though – Water off, drain down.  Undo the back nut and remove the PTFE.  Careful not to lose the back nut through the cupboard back otherwise you’re in for a world of hurt ( I sometime put a zip tie around the back before undoing and leave the tail so it can’t happen.  Don’t try and replace the olive as you normally need an olive splitter to get them off, just put some jointing paste on as detailed above then refit, don’t go crazy tightening!  Too tight is as bad as to loose.

PTFE is for tapered threads which this is not.

In all likelihood it’s probably one of the  causes of the leak by not letting the seal faces meet when the threads made up fully

It seems PTFE on threads is a Controversial Topic

PTFE tape on a compression joint would get you fired where I work. It’s for tapered thread only.

As others have said – remove all tape off the fitting, boss green, job done.