I had a similar problem with my 1985 Coffex project. I flipped the teflon plug but it was badly worn and I suspected some steam leakage even after tightening it. I have the complete rebuild kit from OE and on a second pass through the bag decided to try the same-size black one with a center hole and it fit perfectly. It is a hair thicker, and the old one was wanting me to screw down to the end of the thread range. The hole does not matter.
The picture shows my two measurement gadgets. They both work very well. On the left is a piece of 3/8" brake line with a hose clamp holding a red wire nut to seal the end; another hose clamp seals on the tip of the steam wand. I drilled a tiny hole in the side and insert the filling needle that came with my stand-up bicycle pump. I was able to raise the pressure in the system slowly to about 2 Bar before the safety valve tripped; I then set it down to about 1.5 as my PSat is now cycling from 0.7 to 0.9. How did I measure the pressure? One the right is a longer piece of brake line with a pressure gauge fitted to the end; it slips over the wand. for this one, at lower pressure, I don't need a hose clamp.
Source: my friendly locally-owned auto parts store. The clerk disappeared into the back recesses and came out later with the whole thing assembled for $35. The pleasures of small town life. I should hold court with my espresso machine and treat the Saturday morning crowd to my best!

Hi Pete -

There's an alternative to using the steam wand to attach a manometer. On machines with a boiler cap like the Livietta, you can kludge a setup such as this -


Found it much easier to work with than the steam wand method. And must say your color scheme is growing on me.

I dragged the cap and gauge to the local hardware store and just put it together with what they had and gas spec teflon tape worked well at sealing it. Good thing, as any mention of British or metric sizes seems to irritate them.