The U.S. Department of Transportation has launched an investigation into brake problems in the 2010 Toyota Prius, adding to the Japanese automaker’s woes. The announcement follows reports that drivers temporarily lost the ability to brake on uneven road surfaces, the CBC reports. On Thursday, Toyota acknowledged design problems with the anti-lock brake system on its latest model of the Prius, a gas-electric hybrid, but the company has yet to decide how to inform people who bought them. The model under investigation first became available in May; about 170,000 were sold in Japan, and 103,000 in the U.S. In the U.S. and Japan, about 180 complaints have been lodged about braking problems. This follows a worldwide recall of 4.2 million Toyota vehicles over sticky accelerator pedals.