Never cap off vent pipes. These pipes are vital as they deliver air into the plumbing system. Most people think that the plumbing vent pipe up on the roof is like a factory chimney that exhausts smoke. Vent pipes work in reverse. They deliver air down into the system each time you flush a toilet, drain a sink, or run water into a fixture. The moving water not only pushes air ahead of it as it travels to a septic tank or sewer, but it can also create a vacuum as a slug of water passes a drainage branch within the system.