I have a water leak out of the anti-vacuum valve. It leaks mostly when the machine is off overnight and while warming it up in the morning. I've replaced the anti-vacuum valve with a brand new one without change in the leak. My guess is that something else is wrong and this is just a symptom.

It seems that there should not be any pressure in the boiler when the machine is off (once it cools). Is the solenoid supposed to cut off the water supply to the boiler when power is off? Could a stuck-open solenoid valve cause a leak at the anti-suction valve? My current fix is to shut off the water supply to the unit every night.

Simonelli Appia II 2 Group Manual

Note that I don't own your type machine, but from your description, the valve seems to be operating normally. A typical vacuum valve closes and opens due to pressure in the boiler. If the machine has been off for awhile, like overnight, pressure in the boiler is low and the valve will be open. As the machine warms up, a little water can escape the valve until enough pressure builds and closes the valve. It may seem like a leak, but it's normal operation for these valves. Basically, when pressure builds high enough the valve should close and no water should leak from it thereafter... that is until you turn off the machine, pressure in the boiler drops, and the valve opens again to repeat the cycle the next time you power on.

The question is... Does the valve leak after the machine has already warmed up? If not, then probably nothing is wrong with the valve. The valve has only two positions... closed or open (unless it's leaking due to bad seals) and nothing else on the machine should affect it's operation.

That all makes sense. However the machine is leaking when it is off, overnight. After the boiler cools down there shouldn't be any more water pressure in the boiler and yet the anti-vac continues to dribble water all night creating a huge puddle on the counter and floor. It works properly when the machine is powered on and up to pressure.

I'm unfamiliar with your machine, however, the manual says "water works pressure not to exceed 4 bar max". I assume you have some kind of regulator installed to limit line pressure to 4 bar?

From the symptom of water exiting an open vacuum valve (when machine is off), it would point to a leaking water fill solenoid which is overfilling the boiler at night due to line pressure. Normally the water fill solenoid is closed to the boiler unless it's commanded open to fill the boiler. If it's passing water when it's supposed to be closed, then I can see the symptoms you describe happening. If it is the solenoid, then the piston seal is probably bad, overly compressed, or damaged. It could also be scale causing the solenoid springs not to function optimally, so it would be a good idea to disassemble/clean/inspect the solenoid. Please don't forget to unplug the machine before working inside.

Keep in mind I don't know your machine or how it's plumbed so hopefully someone with more dedicated knowledge will chime in.