A typical air disc brake generates the braking torque by friction between rotor and pads located on both sides of the rotor. Friction causes pad and rotor to wear every time brakes are applied. As a result, a gap called running clearance is created between pads and rotor. Automatic adjuster compensates for the wear and keep the running clearance stable. If any malfunction occurs in the automatic adjuster, brake will not self-adjust and the gap will get larger. In this study, the effect of the running clearances to the braking performance was analyzed. Braking forces caused by friction between brake pads and rotor were calculated from air disc brake’s mathematical model for different running clearances and compared with the experimental results. Both the calculated and experimental results indicate that the braking force is affected from running clearance. Air chamber stroke increases proportionally as the running clearance increase but it has a physical limit. The mechanical advantage ratio of operating shaft is higher at large stroke values but it is observed that the air chamber, which provides the input force is inversely affected by the clearance after 50mm pushrod stroke. Excessive brake gap combined with high deflections caused by high input pressures overcome the operating shaft ratio increase and reduce the output force of the brake.