The complaints are believed to be related to ABS hydraulic control unit faults.
Older Ford Fusion and Mercury Milan sedans are under investigation over complaints of brake problems.
The National Highway Traffic Safety Administration has identified at least 141 complaints of "sudden, unexpected increases in stopping distances" with the 2008-2009 Fusion and Milan.
The apparent brake failures are believed to involve a fault in the ABS system's hydraulic control unit (HCU), which causes an "inability to maintain required braking pressure." Most complaints claim the concerning behavior arises after the ABS system activates as designed on a slippery surface.
"Reports allege that the condition can continue for some time until it corrects itself, but will then reoccur if another ABS type braking event happens while other reports indicate that the replacement of the ABS HCU corrects the problem," the investigation notification says.
During the alleged failures, owners claim the brake pedal goes soft, described as "going to the floor," and the amount of force required to stop the car increases significantly. The NHTSA suggests some complaints claim the vehicle stopped within the desired stopping distance, but in other cases the vehicle is claimed to have rolled through a stop sign or red light.
At least three crashes are associated with the condition. A recall, if demanded, could affect nearly a half million vehicles in the US.