Compression fittings. AKA "Wizards of NOS plastic pipe joints
A mystery to some, loathsome to others and lovely things to the rest of us for loads of reasons.
A compression joint comprises of the female, an olive and a male tube nut.
To make a new joint;
Cut the pipe as square as possible using a brand new craft knife or Samurai sword and cut it against a lump of wood (a cutter is available but isn't all that good, snips and pliers etc certainly won't work so get a good knife, when cutting it you'll suddenly cut through it after putting what seems like a ton of pressure on the knife, if you don't cut against something like a block of wood then be prepared for a trip to casualty about now)
Slip the pipe nut onto the pipe and now put an olive on the pipe, they can be fiddly, tight little sods, don't force it and as long as your cut was clean it will go on once you get it lined up properly.
Slide the olive at least 4mm up the pipe.
Put the pipe inside the female and make sure the pipe finds its way into the little recess, Hold it tight and slide the pipe nut into the female half, add a tiny drop of grease, just enough to help lubricate the thread, if you fill the first thread that’s almost too much. DO NOT USE SEALANT !
Push the tube nut in and turn it clockwise until it engages with the thread.
Tighten it by hand or with very light spanner pressure until you feel it stop.
Mark the female and pipe nut with a black marker across two of the flats.
For 4mm pipe tighten the 8mm pipe nut between 4 and 4.5 flats
For 5mm pipe tighten the 10mm pipe nut between 5.5 and 6 flats
About now it will have gone from "feeling stiff" to "quite hard to turn actually" in the space of the last half a turn.
This is tight. To check just undo the nut, tap the pipe with a spanner while pulling on it until it comes out.
The olive should be tightly gripping the pipe but not crushing it and the pipe should protrude from the olive by 2 or 3 mm.
Olives are cheap so have a go yourself, at least crush the pipe a few times so you really know how tight is right.
If you’re going to get a pipe coming out then it'll be the 4mm high pressure because it has less surface area for the olive to grip and is more susceptible to being under tightened. Tighten it right and it won't come undone, I've got one and its held system pressure for a year, only being depressurised when I refill the bottle, same olives.
To refit the pipe then just poke it back into the hole and carefully do the tube nut up finger tight, you have to be careful the pipe finds its little recess before getting the spanner on the job so you don't force the olive off the pipe. Once its finger tight then use the spanner to tighten it. 4mm by 1 flat and 5mm by about 1/2 a flat, you'll feel them suddenly go tight at this point so stop. These things seal incredibly well and don't need to be "grunt" tight
When used between the same two halves of a joint they can be done up and undone hundreds of times and cost buttons to replace if they do wear out.