To operate properly the cap must be loose. They do go bad and leak. When that happens you must replace it. You can tighten it to stop the leak but if you develope air problems it must be replaced. If this is new your contractor should replace it for free.

Thanks for your reply

When he left the first day after the install this one was leaking and another one (not on the boiler, but at another place) - both new. I told him that I had closed them to stop the drip, and the response was something like, that is fine; just remember to periodically bleed them.

What is the consequence of not replacing? I have so many other issues to deal with as an aftermath of the job that I am tiring and performing triage on what is essential versus an inconvenience.

if he put the auto bleeders in, he must of thought you needed
them. So if you need them they should be left open. if you are going to open them just once in a while then he could have put coin bleeders in for 1/10 the cost. If two of them started
leaking right away then it sounds like you might have too much junk in the system. maybe the system needs to be flushed
out. later paulbm

The consequence is air will be trapped in your system and stop the circulation of heating water. That's the short answer. Air is an enemy of hot water heat. Your contractor should replace them. As paulbm said, you could have crap in your system that got stuck in the valve seat. You can leave them alone and see what happens or you can remove the caps and press down on the stem to see if it clears. If your sytem develops air problems get them replaced. You should not have to bleed your own system because of a lazy contractor. Believe me, he charged you for them and they are under warrenty.

25 years in the field and i think every air vent leaks or just doesn't work after a very short time, and know it seems any brand that you use leaks almost immediately, poor quality control i guess, however it seems that the 3/4" air vents are much better and more reliable than the 1/8" so good luck

They are the TACO Hy-Vent automatic air valves: one atop water tank and other atop highest point in system.

OK, so I need to tell my contractor that there is gunk in my system, which has caused my shower and tub faucets to begin dripping and which is causing these air vents to leak. Got it! Thanks so very much for your replies. Onto chapter #5 of the boiler replacement saga . . . .