I have a problem with sight glass level. A boiler is equipped with manual water adding. When I adding water to the boiler manually in the sight glass water rising to some level and stops despite the fact I've not stopped the water. If I do not stop adding water it will flood the boiler. When I unscrew the valve cartridge from the upper valve on sight glass, water in the glass raising and water speel from that valve. Manually I drain the water to the correct level, then install back valve cartridge. As soon as I install back cartridge, the water in the glass raising up and fill up the glass. When I remove the cartridge, water level lowering to correct level. Both valves on the glass I cleaned and water from boiler have a clean connection with glass tubing. Can you describe the problem, please?