(February 6, 2010) The National Highway Traffic Safety Administration said today that it is opening a formal investigation of the 2010 Toyota Prius Hybrid to determine whether the vehicle suffers momentary loss of braking capability while traveling over bumpy surfaces.

Toyota acknowledged design problems Thursday with the brakes in its best-selling Prius, adding to the woes for the Japanese automaker as it reels from massive gas-pedal recalls in the U.S.

Toyota said Thursday it found design problems with the antilock brake system and corrected them for Prius models sold since late January, including those shipped overseas. Toyota says it conducted a software fix to address reports of slipping brakes.

But the world's biggest carmaker didn't notify customers at the time of the change. And it says it is still uncertain how to handle the thousands of Prius vehicles that were sold before the fix.

The problem occurs in the third-generation Prius, which went on sale in the United States, Japan and Europe last year. Transportation agencies in the United States and Japan have been compiling dozens of complaints that the car's brakes give way under certain conditions. But the company said it is still investigating how to inform customers who had bought them earlier. Nothing was decided on that front for Prius gas-electric hybrids sold outside Japan, according to Toyota.

More than 180 complaints about braking problems in the Prius — the world's top-selling hybrid — have been reported in the U.S. and Japan. Those complaints come amid a global recall of nearly 4.5 million vehicles for faulty gas pedals.

"We are investigating whether there are defects in the Prius," Toyota executive Hiroyuki Yokoyama told reporters at the automaker's Tokyo headquarters.

Paul Nolasco, a company spokesman, said the time lag for brakes kicking in felt by drivers stem from the two systems in a gas-electric hybrid — the gas-engine and the electric motor. When the car moves on a bumpy or slippery surface, a driver can feel a pause in the braking when the vehicle switches between the traditional hydraulic brakes and the electronically operated braking system, he said.

The brakes start to work if the driver keeps pushing the pedal, but the driver may momentarily feel they aren't working, he said. Fixing that included a software programming change, he said.

Whether a recall is in the works for the Prius is still undecided, according to Toyota, but Japan's transport minister urged the company to consider it and is ordering an investigation.