This is the part I hate about shop maintenance.

[LCHIEN]

Fixing stuff that breaks for no apparent reason.

Was working in the shop and hear air hissing pretty loud. Near the compressor. Which is of course hard to get to.

So I finish what I'm doing and investigate, awkward to reach but I can feel air blowing on my hand which means its a serious leak,
So I have on the QD of the compressor a 4-way manifold so in a close space I have at least a dozen or more junctions and threaded joints.
I've got three lines into the manifold all with QD. I'm twisting and turning all the QD to see if one is leaking because its not seated well, then I start swapping them.Still big leaks
unplugging one or another and not resolving it fast. And its a mess so I take an extension 50-ft PU hose and connect to the compressor. No leak. Then connect the manifold and the three hoses, I can still hear it, but now I'm able to dunk the manifold into the bucket of water I've brought to the party. Three leaks. Looks like one hose to QD nipple leaks slightly so I note that. And two QD females on the manifold are both leaking badly from around the base, it looks like.
Now this manifold I recall I picked up about 15 years ago when they were cleaning out some office and lab space, I found it abandoned so I gave it a home and used it for many years.Now its leaking like a sieve, kind of all of a sudden. It's a real *******. May have come from our European office because as it turns out all the brass fitting hex are 14 and 17 mm, not SAE. But the fittings are 1/4 NPT and Milton/industrial style at the ends.I spend a stupid amount of time trying to get it apart (this is where I find its part metric) and that I need a thin 17 mm wrench to get the female QD off. All wrenches I have are too thick.
I look at buying a tappet wrench but they're expensive and so now get a wild hair and find that Amazon, while not having cheap thin individual wrenches have 3, 4, and 5-way manifolds. Curiously the 5-way is the same price as the three way and both are cheaper than the four way. So for a little more than $16 I have a 5-way manifold. coming Monday.

Being an engineer I continue to mess with it and manage to find (I didn't know it) a threaded joint between the Knurl below the QD ring and the hex. I take off the top of the QD and now Its easy to get a wrench on the base hex. But not easy to get off. There's some kind of hard glue been applied to these threads. No Teflon tape, no pipe dope.The QDs are all off now after breaking the sealant (which in two of the four had failed).
Chase out the 1/4 NPT female holes with a tap to remove burrs and old adhesive. Oddly the Male NPT on the QDs was relatively short. Never seen them that short.
I think I could put some new QDs on this and make it work again. But the other is on the way. and I don't have 4 male NPT QDs and they'd still cost more than the new manifold at 5 bucks cheapest Amazon fitting.

Near as I can figure out the seal they put on the threads all failed after so many years for two of them to go at once. I really don't know if it was bought or made and why the mix of metric and SAE. Oh yeah, even the 1/4 NPT plug in the end has a metrics hex recess for Allen wrench.

Just a mystery.

• why its metric except for the 1/4 NPT - where did it come from
• why did they use a hard sealant
• why did they have multiple failures all at once
• Why are the 1/4 NPT males so short... were they modified in some way - I was thinking they were cut down to not sit so deep in the manifold threaded holes? perhaps they bottomed out before they sealed and were cut off and sealed with some odd stuff?

I haven't drained the compressor in quite a while (lazy), I probably drained over 24 oz. of water from it.(bad on me)
And I reapplied Teflon tape to the slightly leaking hose stud.

[d_meister]

[*]why its metric except for the 1/4 NPT - where did it come from

And I reapplied Teflon tape to the slightly leaking hose stud.

It may not be NPT, just looks like it. International versions are very similar and can be interchanged. I heard that the British invented self-sealing tapered joints during WWII, but that could just be BUL (British Urban Legend)
Regardless, British Standard Pipe is the world ISO standard for that type of joint: From Wikipedia

"British Standard Pipe (BSP) technical standards for screw threads that has been adopted internationally for interconnecting and sealing pipes and fittings by mating an external (male) thread with an internal (female) thread. It has been adopted as standard in plumbing and pipe fitting, except in North America, where NPT and related threads are used."

As to using teflon tape, I hope you're using Teflon sealant tape. There are different types available, even at the BORG. The common white teflon tape is best used as a thread assembly lubricant for properly machined and matched pipe threads. The other two commonly available are the yellow teflon tape for natural gas, and the heavier gray teflon sealant tape. There are other brands that are even thicker (better) available commercially. On my new list of things to do, since reading your post, is disassembling all of my airline connections and sealing them with either Permatex hardening, or the gray tape. Depends on which is easier to find, around the shop When I shut off the valve on my compressor manifold and shut off the power, the system leaks down over time. No hissing, though. I think the white teflon tape may be at issue, or partly so.
A couple of things I learned about teflon tape, over the years; never use it to seal diesel piping. It WILL leak. And, the heavy teflon sealant tape works really well on troublesome water plumbing leaks. I came across a roll of the heavy tape years ago on a yacht I was working on after trying to seal the same water joint three times. The parts were all new, but rough marine bronze. Fixed it.
I don't know what I'm going to do with all of those rolls of Harbor Freight teflon tape laying around. I used to throw a sleeve of ten rolls in the basket when shopping because it was so cheap, Maybe plant stake tape?
Here's the products:

https://www.oatey.com/products/thread-sealants/tape

[leehljp]

I use a Blue HD teflon tape and it does well. A little expensive for teflon tape but it does well. I had leaks on my lines too but after getting some good fittings and using the blue teflon tape, It has stopped! I love a non-leaking hose and fittings!

AS to thin wrenches: I picked up a couple of sets of metric wrenches overseas that were/are about 3/32 thick. Not quite 1/8 in. But they are stiff and don't give.

Two weeks ago, and this past weekend, I ordered several "organizers" for wrenches and sockets. I had the sockets just thrown into two drawers of my tool storage boxes: 1 for metric and 1 for USA/inch; same for wrench sets. This weekend was when I discovered I had two sets of those flat metric wrenches. Organization does help!

[LCHIEN]

Well, must be a success. In fact, I guess that manifold was leaking for a while. The compressor used to cycle every 20-30 minutes, it's been 2 hours and hasn't cycled yet.
Installed the new 5-way manifold without a problem, I leak tested it in the bowl of water before installing, just to make sure.
Oh, Just cycled.