Please verify when replacing a hot water pressure bladder that the air needs to be removed from the raidiaters and that the same type of air bladder just screws back on the same nipple. Also the bell shaped valve next to the air bladder has a plastic nut on the bottom and when it is removed a piece of coroded pipe tubing is there with a hole in it . What does that do?
The bell shaped valve is a boiler feed valve - just a type of pressure reducing valve. Usually runs at 12 pounds, but some systems will use different pressures. The plastic nut is attached to a cylindrical screen that keeps debris from getting into the workings of the valve. Sounds like it hasn’t been cleaned in a while.
Rads should be bled as necessary. I usually bleed at the beginning of the season, any time the system is opened, or one becomes air locked.