Before I go into deep detail here I am putting this question out just in
case there may be something I overlooked.
I installed this boiler and controls (complete) 35 years ago and maintained
it ever since, I am familiar with pressures, flows, etc.
Last winter I shut off my supply valve to the regulator 12#, after shutting
down the boiler my pressure dropped off to 0 psi.
I visibly checked through 2 zones and found no leakage, it kept doing this
and still does.
I removed the automatic vent valve above the flow valve, also removed the
expansion tank.
Plugged the 1/8th inch outlet and used the 1/2 inch to connect a line adding
21# of air. This kept solid for 24 hours,
after visual checks and pressure test I am 100 % confident no leaks in the
system are present

There is also no indication of leakage in the boiler, I removed skin,
cleaned and checked all I could see,
I did not want to remove the outlet hood above the boiler tubes as it was
cemented to the water jacket.

I have been hearing a slight gurgling for sometime, inside the boiler. I am
aware noises can travel, however I am certain this gurgling is from the
boiler internals.
All air has and is purged from all the vent valves in the system. along with
the automatic vent valve.

Now here's a kicker that just happened today. Yesterday I add water to bring
up pressure to
12# (from 7#) it went down to 7 again as it has been, and then it
actually went back up to 10lbs...

Valves around regulating valves are closed and do not leak through..
Is it possible I somehow have air trapped in the boiler cavity that will not
vent?.
I am trying to keep from spending $3- 4000 for another boiler unless
absolutely necessary.
If I need to my next question I will post in greater detail
thanks