The pilot of the airplane reported that, during the landing roll, the airplane was not slowing as expected. The pilot firmly applied braking action with no corresponding slowing action from the airplane. When he and his passenger saw the departure end of the runway approaching, they both applied as much brake pressure as possible without any effect. The airplane departed the runway and came to rest in a ditch, resulting in substantial damage to the engine mount. The airplane’s brake system was inspected by a mechanic who reported that the left side brake functioned, however it had a minor amount of air in the system and may not have had the same working pressure as the right-side brake. The pilot reported that there were no other preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.