ATA 29

Emergency Hydraulic Pump
Are you noticing that your aircraft's emergency hydraulic pump is cycling too often and norma servicing does not seem to help? The thermal relief valve may be the culprit.
If there is no evidence of a hydraulic leak, first check the emergency hydraulic pump accumulator nitrogen servicing. Be sure to observe the procedures outlined in Westwind 1124/1124A Maintenance Manual Chapter 12 to ensure the accumulator piston is bottomed out. If this is correct, check the accumulator for proper operation. This can be accomplished by attaching a pressure gauge to the emergency accumulator nitrogen servicing port. With the port oen, run the emergency hydraulic pump and witness the atached gauge reading full system pressure. If it reads correctly, then the thermal relief is likely leaking internally.
The thermal relief valve has two functions. It provides pressure relief if the system pressure becomes too high, and it has the only check valve in the system to prevent hydraulic fluid from returning to the emergency pump. If either side of this valve fails to operate properly, the cycle time will be high.