a dual - function deflation valve for hand - held blood pressure measuring devices with a finger - depressible valve actuating lever which has a detent mechanism capable of retaining the lever in the fully open valve position , for a quick release of residual air pressure from the measuring cuff . the detent mechanism has two detent members with cooperating cam formations defined by the valve housing and valve actuating lever , respectively , either on coextending wall portions of both , or on a wall portion of the actuating lever and on a spring member which is mounted in a recess of the housing .