GOING 35MPH DOWN STEEP GRADE, STARTED TO APPLY BRAKES TO SLOW UP; RIGHT TIRE LOCKED UP & THEN LEFT LOCKED. STARTED TRUNNIG WHEEL; TOOK FOOT OFF BRAKES, BUT WHEELS STILL LOCKED. VEHICLE DID NOT SLOW UP. HIT GUARDRAIL HEADON & DRIVER'S AIR BAG DID NOT DEPLOY. TOWED TO DEALER & TOLD COULD NOT FIND PROBLEM. *AK