308C CR Excavator Hydraulic System – Main Hydraulic Pump Air Pressure – Purge

Procedure for Releasing Air Pressure

When the main pumps have been serviced or the hydraulic oil has been replaced, remove the air from the main pumps. Remove the air from the main pumps in the following manner:


    Care must be taken to ensure that fluids are contained during performance of inspection, maintenance, testing, adjusting and repair of the product. Be prepared to collect the fluid with suitable containers before opening any compartment or disassembling any component containing fluids.

    Refer to Special Publication, NENG2500, “Caterpillar Tools and Shop Products Guide” for tools and supplies suitable to collect and contain fluids on Caterpillar products.

    Dispose of all fluids according to local regulations and mandates.


    To avoid system damage, make sure that the hydraulic pump is properly filled with hydraulic oil before start-up.

    Illustration 1 g00539166

    Typical example of the pump compartment

  1. While the engine is stopped, remove the vent plug on the top of the main pump. Fill the main pump with oil. Install the vent plug. Tighten the vent plug to a torque of 36 ± 5 N·m (27 ± 4 lb ft). Start the engine. Place the engine at low idle position. Raise the boom and hold the boom in this position.
  1. Stop the engine and slowly lower the boom until the bucket is on the ground. This pressurizes the hydraulic tank.
  1. Slowly loosen the vent plug until oil begins to flow from the opening.
  1. Tighten the vent plug to the specified torque.