I was stopped in the left hand turn lane, then accelerated to cross 2 lanes of traffic. I then let off the accelerator, and applied the brakes to stop, and the brakes did not function. I then rear ended a vehicle in front of me due to the intermittent loss of brakes. It may have also been the propulsion system kept going. Either way the vehicle maintained the same speed of about 15mph until I collided with the vehicle in front of me which stopped me. The brakes worked fine again after the collision. There also was no Forward Collision Avoidance system that was triggered in any way. This vehicle is a Rental Car.