I am working on a steam boiler that's air vent leaks water. Also, the boiler bangs when heating. I find the boiler overfilled. Therefore I drain the water out of the boiler. Next, I replace the sight glass that's difficult to see through. Then I flush out the shutoff valve under the boiler's sight glass. The union past the autofeed I open, looking for why the boiler was overfilled. Troubleshooting why boiler overfilled. Water slowly leaked past the autofeed solenoid. Finally, I refill the boiler and test the boiler. Banging went away, and so did the air vent leak.