I have an old one pipe steam system with a Hoffman #75 vent at the end of the loop. My system actually works quite nice in general, I have tuned it up and balanced it over the years to much success. The issue that I just started having about 6 months ago is that my main vent is no longer closing and it requires a small 'smack' on the side in order for the plunger to completely seal the vent and close. The pin does rise into the orifice but it just doesn't seem to seal 100% and I get a nice solid steam jet until the boiler can finally overcome the vent release and get up to 3psi to cycle properly.
I thought that it was a scaling issue or something but then again, since the water is steam, there really shouldn't be any minerals in it to actually build up scale.
Any thoughts? I figured the vent would last more than 1 year? Should I take it off and soak it in lime-away for awhile? Also, the vent is perfectly vertical so it isn't a 'slanting' issue or anything.