I've been chasing a very elusive leak on the coolant plumbing for my induction furnace, desperate to find it before I added the antifreeze when it will only get worse. Very simple bit of 1" bsp pipework in stainless. An elbow, a reducing bush, a brass non return valve and an adaptor to MDPE poly pipe all running vertically. After a few minutes of running there would be the slightest weap dripping off the bottom fitting - so small I could ALMOST convince myself it was condensation. Carefull drying and watching still couln't see where it was coming from. Stripped it all down three times and remade the joints (this involves draining the system). At various times heavy ptfe tape, ptfe twine, loaded ptfe paste - still leaks.
Finally dried it all off and sat watching it all under a magnifying glass and bright light.and low and behold its coming out of the face of one of the spanner flats on the brass one way valve. It's a bally porous casting not a joint after all !!!! Just replaced the one way valve, sweet as a nut. Only taken about 12 hours out of my life to solve this
AWEM