REVERSE PULLING OUT OF DRIVEWAY AT SLOW SPEED THEN STOPPED. ON DRIVE POSITION, VEHICLE JERKED FORWARD WHEN GAS PEDAL WAS APPLIED. THIS HAPPENED TWICE AT SAME SITUATION ABOUT SAME TIME.THEN AT THE FREEWAY, IN STOP AND GO TRAFFIC VEHICLE BRAKES FAILED AND HIT A BIG-RIG TRUCK. NO WARNING LIGHT THAT CAME ON. I PRESSED FOOT PEDAL HARD BUT VEHICLE DID NOT STOP AND CONTINUED ROLLING UNTIL IT REAR ENDED THE TRUCK. LUCKILY OUR SPEED WAS 0-5 MPH AND NO INJURY TO MY 2 PASSENGERS. CHP ASSISTED US. VEHICLE WAS NORMAL WHEN PULLED OVER TO THE SHOULDER. I HAD IT TOWED TO NEAREST MERCEDES BENZ DEALERSHIP. PER SERVICE MINDER, VEHICLE UNDERGO REGULAR MAINTENANCE AND IS SERVICED BY SAME DEALERSHIP.