AFTER REBUILDING ENGINE, EMPLOYEE REMOVED SAFETY CABLE ON DUMP BED, STARTED ENGINE & MOVED IT FORWARD. INSUFFICIENT OIL IN RESEVOIR CAUSED DUMP TO SLAM DOWN BEFORE HYDRAULIC PUMP COULD BRING ENOUGH PRESSURE TO HOLD IT UP. EE REPORTED INCIDENT IMMEDIATELY, BUT DID NOT SEEK MEDICAL ATTN. UNTIL 6/13/05. EE SUFFERED A FRACTURE TO THE 12TH VERTEBRA, HE IS ON MODIFIED DUTY.