I removed the pressure from the system. Well, mostly. I got sprayed with water when I changed out the automatic air vent on the manifold where the expansion tank resides. With the new air valve in position I checked the air pressure in the expansion tank.

20 psi. How the heck did that happen?

With the pressure dropped to 12 psi the boiler is now at 19-20 pounds. I really don't know how that tank was at 20 psi. I checked it before I installed it and it was at 11 psi. I hit it with some air and rechecked it. I thought I saw 13 psi. I must not have had the pressure gauge on correctly.