Posted by Daryl Hickman on June 22, 2000 at 13:22:00:
In Reply to: Questions posted by Educable, but green on June 22, 2000 at 12:36:30:
Actually...
The object of venting a piston or a rotary valve is to create an "open circuit." The open circuit allows the player to pull a slide with out creating a vacuum. In piston valves, a hole is drilled through the side of the piston. When the slide is pulled, air enters through the bottom of the piston, goes through the newly created hole and into the slide. When the slide is pushed in, the reverse happens allowing the air to escape through the bottom of the piston. With rotary valves, a hole is drilled into the rotor casing. Generally, venting is performed on front-action instruments and on slides that are likely to be manipulated during play. Most commonly this is the first valve slide. The first valve slide is pulled on combinations using 1 and 2 because thoser notes are generally sharp. There are plenty of arguments both for and against these precedures.
The one that makes the least sense is that the valve will leak. When properly performed this isn't likely to occur. A hole in a rotary valve casing is susceptable to foreign matter entering the casing, however since most people don't clean their instrument often enough there is a better chance of larger objects entering from the leadpipe.