I found information on the expansion tank noted in the subject line on your web page.
Automatic air vent valve leaking water.
Float type automatic vents on hot water heating systems are normally left open to automatically purge air from the system.
If you still have air in your system after that point you have a system leak which is causing the fill valve to add water and entrained air.
Guide to repairing an auto air vent aav.
This is the kind i have on my gas furnace.
There is water coming out of the top brass venting thing above the tank.
Here we explain how to repair or replace both manual heating system air bleed valves and automatic float type air vents that are leaky.
An automatic bleeding valve or air release valve arv is a plumbing valve used to automatically release trapped air from a heating system.
Air vents automatic air vents should be isolated after original fill of closed hydronic systems.
Based on tapping the expansion tank it does not seem waterlogged bottom 2 3rd of tank sounds hollow or empty.
The taco automatic coin vent installed on water or steam radiators the taco automatic coin vent is the ideal air purging device.
Leaky hot water heating system vent repairs how to repair a leaky hot water air vent or bleeder valve.
Alot of times a leak can be traced back to the relief valve opening.
It has an air purger with an expansion tank screwed into the bottom and an automatic air vent on top of the air purger.
Taco automatic air vents include.
Air or other gas may collect within plumbing for water delivery systems to taps and basins particularly with good main supply pressure this air is usually flushed through with the water flow and does not cause a problem although in some hot water.
The air vent is leaking some water.