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Steam Vacuum

I just installed a Weil McLain LGB 7 steam boiler to replace the same boiler I installed 18 years ago. The install went fine. It's a two pipe system in a large house. The only two vents in the system are located on two main returns back at the boiler. The vents are high capacity which I replaced. When I started the boiler I noticed it would create a vacuum and suck the water from gauge glass and turn off boiler on low water,once the system cooled down the water filled gauge glass and turn on boiler.The boiler I removed worked all that time and never had an issue like that. What Changed? Why would I need a vacuum breaker in system after all these years?All returns are newer and water is getting back to boiler? What am I over looking?Thanks,Joe