Having a lubrication issue with an '03 735SL-Y (GMK 3092):

We were getting Alarm 1021 (Lubrication Pressure Operated Cut-Off Switch Defective). We replaced the pressure switch just in case, but this seems to be an issue with the exhaust valve on the pump not venting the lines back into the res when the pump stops, keeping the pressure switch closed. The pump produces 24bar, pressure switch closes, pump turns off but the line pressure hangs around 18-20bar.

After taking the pump apart and cleaning it out, the exhaust valve is still not working 100% but we don't have a replacement right now and we're no longer getting 1021. The alarm did change to: Alarm 1009 (Defective Lubrication) which lists the following possible causes:
  • Low Tank Level (Tank is full. Unplugging float switch to simulate a low tank level gives alarm 2055 like it should)
  • Oil leakage in the system (No obvious leaks, system holds pressure longer than it should)
  • Air in the lubrication system


I'm leaning more towards air in the system since it was just disassembled. The maintenance guide recommends running the pump manually to clear air from the lines, which we've been doing without success.

How is the control determining there is air in the system or that there is a leak? It seems like the only related sensor is the pressure switch.