bleed valve

noun
  1. a valve for running off a liquid from a tank, tube, etc, or for allowing accumulations of gas in a liquid to blow offAlso called: bleed nipple
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012