VEHICLE - EATON HYBRID FORD F550DR BUCKET TRUCK VIN [XXX]. DRIVING ON IH 35 AT APPROX. 50 MPH, TRAFFIC CAME TO AN ABRUPT STOP. I HIT THE BRAKES ON THE HYBRID TRUCK AND THE BRAKE PEDAL WENT TO THE FLOOR ( I WAS PUSHING THE PEDAL VERY HARD ), THE FRONT AND REAR TIRES DID NOT LOCK UP NOR DID I FEEL THE BRAKE PEDAL PULSATE INDICATING THE ABS HAD BEEN ACTIVATED. I HIT A VAN FROM BEHIND HARD ENOUGH TO PUSH IT FORWARD AND THEN HIT IT A SECOND TIME AND PUSHED IT UNTIL THE VAN BROUGHT US TO A STOP. I BELIEVE THE BRAKES FAILED, IT OCCURRED ONLY ONCE, RESULTING IN A COLLISION. I THINK THAT THE ADDED WEIGHT OF THE BATTERIES AND THE AERIAL TOWER ON THE 550 CHASIS IS MORE THAN THE BRAKES ON THE TRUCK ARE ABLE TO HANDLE. THE TRUCK WAS SENT FOR BODY REPAIR AND THEN TO RIATA FORD IN MANOR TX FOR THE BRAKES TO BE CHECKED. ACCORDING TO THE QUALITY CARE REPORT " TEST DRIVE VEHICLE AND BRAKE SYSTEM IS OPERATING PROPERLY. " NO PARTS WERE REPLACED. *TR