Brompton Seat Post BungBrompton Seat Post Bung Brompton Seat Post Bung Are you looking for the seat post bung by which you can sit easily and ride your Brompton bike? Then this for you. The seat post bung from Brompton is a rubber bung that fits into the bottom of the steel seat pillars. Posts stops the Seat post from clunking on the ground when the seat post is lowered, and It also helps to prevent the bike from rolling when rack wheels are fitted and you don't want it to!