Abstract: The apparatus comprises a lower sensor-holder head (40) and an upper sensor-holder head (72) each located at the center of an annular gasket (20, 70). Either (or both) of the sensor-holder heads are received in a space extending along the center axis of the gasket and are axially movable between a retracted position and an extended position. Preferably, one or both of the gaskets has stepped concentric rings for adaptation to different test object diameters. In operation, an object to be tested such as a wheel rim (C) is sealed between an upper gasket and lower gasket to define a space inside the wheel rim which encloses the sensor-holder heads, and to define a space enclosed between the outside of the wheel rim and an airtight bell chamber (66) which is pressurized with air or testing gas. Any leakage through the wheel rim is detected by one or more sensors on the heads.