Lindsay Lohan's brakes couldn't have failed last month -- moments before she slammed her Porsche into an 18-wheeler -- because sources tell TMZ, Porsche itself tested the car after the crash ... and the brakes passed with flying colors.
TMZ broke the story ... Lindsay blamed faulty brakes for the accident.-- telling friends she slammed on the brakes when the truck allegedly cut her off ... and nothing happened.
As it happens, we're told Porsche caught wind of Lindsay's excuse and contacted the company that rented Lindsay the vehicle -- asking if it could formally test the car.
According to sources, the rental company obliged -- and Porsche ran its tests, finding the brakes that had been installed on the car were A-OK, and showed no evidence of malfunction ... which blows Lindsay's bum brakes excuse out of the water.
A rep for the Santa Monica PD had no comment. A rep for Porsche also had no comment -- but did say they're cooperating with authorities. Calls to Lindsay were not returned.