An airline lubricator characterized by the provision of a backstop which limits and controls the contour of the lubricator's flow restricting elastomeric flapper, whereby higher flow capacities may be achieved without sacrificing flapper service life, and by a sight tube seal plate that supplies lubricant to and from the sight tube via respective passages at least one of which has opposite ends thereof configured for communicating with transversely offset passages in the body of the lubricator head.