CERTAIN TEREX TELELECT DIGGER DERRICKS AND AERIAL DEVICES EQUIPPED WITH CHECK VALVES WITH P/NOS ENGRAVED ON THE VALVE OF; 1904F, 1908F, 1910F, AND 1910R. THESE VALVES CAN FAIL WHICH CAN CAUSE THE PARTS TO MOVE THROUGH THE HYDRAULIC SYSTEM OR WILL ALLOW A VACUUM TO FORM IN HYDRAULIC LINES IF THE PUMP FLOW SHOULD STOP. THESE DEVICES ARE INSTALLED ON TRUCKS TO TRANSPORT THE AERIAL DEVICE OR DIGGER DERRICK TO THE WORK SITE TO FACILITATE WORKING ON ELECTRICAL POWER LINES AND COMMUNICATION LINES OR TO TRIM TREES.