I have a FHW heat system with an Amtrol air purger. On top of the vent there is a small plastic red cap. How tight should this cap be? Is it normal for a small amount of water to leak out of the vent when the cap is not tight? Last year I had the system serviced because water was leaking. The service man told me to tighten the cap down all the way, but when I do this, it does not seem to purge properly.