If you loose the air bubble and have to drain something is wrong with
your system and it will not have expansion capabilities and can
overpressure. Water in it when cold - tank is to small, or it does not
drain properly- blockage, or bad autrotrol valve, or bad auto feed, over
filling system. Pressure - altitude gauge may be bad if you rely on
that for monitoring and manualy feed. Often pressure relief will
release on even a partialy full tank from overpressure on a fully hot
boiler, your relief valve could be bad also. Maintain overpressure and
boilers do not last. Start with a new presure gauge and set boiler to
right level.