I tore down my four-year-old Synchronika recently because I needed to replace the steam boiler gasket, among other routine annual maintenance tasks. Being ham-handed, I managed to create other problems, including a couple of leaks that I tracked down and fixed. However, there is one leak that has me stymied. A small amount of water (50-100 ml?) finds its way underneath the machine when the plastic reservoir is inserted after an hour or so. The reservoir itself does not leak when removed from the machine. I've placed it in a bowl for an hour and nothing drips. Water pools at the bottom of the "Water Tank Guard Rail," which is the shell into which the reservoir is slipped in to, around the white plastic receptacle that the metal "union" of the reservoir plugs into. So water collects directly under the reservoir, eventually making its way beneath the machine.

Seems to me the issue has to be one or all of the three gaskets that are upstream, that connect just above and below the reservoir itself. Does that look likely? Or am I missing something?

My first thought is that the red o-ring on the outside of the reservoir's metal valve is nicked. It's sliding in and out of the receiver quite often, it's likely to eventually get a nick. Then the head pressure in the reservoir allows the water in the receiver to flow up past the o-ring, along the bottom of the reservoir and drip into the holder.
Just a guess... Perhaps even lubricating the o-ring with molykote 111 or something could help it. Does it look perfect, still? Is it still soft?

The lower tank coupling has a habit of hairline cracks, especially if its the older white plastic as opposed to the newer grey plastic, just replace it using the same nut to hold in place. Part number P6020.k4

Thanks to both of you. The o-rings look good to me, so perhaps that white receptacle needs to be replaced.

I'll put some molycote on the gaskets, just to see if it makes any difference.

I wouldn't have believed it a few weeks ago, but kitt's suggestion was spot on. I replaced the coupling with the newer gray model and the leak has disappeared. I took the old white coupling, covered the two barbed connections with my fingers, and blew into the opening that screws into the base of the reservoir. Sure enough, air/water came out.

THANKS!