BULLGANG CREW FILLED DUST TANK WITH SACK DUST PRESSURIZED TANK TO LEVEL DUST. FOREMAN WENT TO BLED TANK OFF. EE WAS SETTING ON TANK AND LOOSENED BOLTS FOR TOP LIP BEFORE WAS BLED OFF. LID FLEW OPENED AND AIR PRESSURE CAUSED EE TO FALL OFF DUST TANK SEAT TO GROUND. BONE CHIP OFF L-1 VERTEBRA.