Abstract: Method and apparatus for indicating on a pressure differential gauge whether air leakage in a compressed air line is within a predetermined acceptable level, the gauge has pressure-responsive and manually shiftable pointers and is connectable in the air line across a first restriction in the line for indicating by the pressure-responsive pointer pressure differentials across the first restriction, and a second restriction temporarily insertable in the line downstream of the first restriction for passing air at a rate equal to the permissible air leakage from the line, and discharging air through the second restriction and indicating by the manually shiftable pointer the position of the pressure-responsive pointer during such discharge, the manually shiftable pointer after removal of the temporary restriction indicating by the relative position of the pressure-responsive pointer on charging of the line whether leakage therefrom is within acceptable limits.