The probe by the Office of Defects Investigation, under the National Highway Traffic Safety Administration, will cover around 420,000 units of the F-150 trucks from the 2013 and 2014 model years, according to document posted on the ODI website, and as first reported by the Associated Press.

In a letter to Ford (F) that was dated March 3, 2016, investigators said they have received 35 reports so far about alleged incidents of “reduced brake effectiveness,” with brake fluid leaking from the master cylinder into the truck’s brake booster. In some cases, this caused the brake pedal to reach the floor with “complete loss of brake effectiveness.”

Thirty-one of the reports were received in the last 12 months, including 20 in the last 7 months, authorities also state. All of the complaints involved trucks equipped with 3.5L engines, representing about 36% of all sales of the F-150 trucks in the 2013 and 2014 model years.

While it’s still in the early stages, the investigation could impact a huge portion of drivers, since Ford’s F-Series trucks are the best-selling vehicles in the U.S. for 34 straight years. Last year, the F-Series sold 780,354 vehicles, and the F-150 accounted for around two-thirds of sales, according to the AP.

In February, Ford China recalled 191,368 vehicles under the company’s Kuga and Focus lines to further inspect brake booster seals. The recall affected vehicles manufactured in China between 2013 and 2014.

A Ford spokesperson told Fortune said the company would cooperate with the investigation. “We take the safety of our customers very seriously, and we will cooperate with NHTSA on this investigation, as we always do.”