Most likely cause of leak is worn out valve rotor seal. It is common that the Pump HP test passes while HP System fails on the same rotor seal, because the valve is in a different position in the two diagnostic tests.

Follow the instructions in the User guide “Replacing a valve rotor and or valve stator".

It might take multiple iterations of degas to reach acceptable values. Changing the valve rotor seal introduces larger amounts of air into the system. It is helpful to switch solvent A tube to solvent B bottle for degassing. Solvent exchange with A and B tubes in corresponding solvents is needed afterwards.

Evosep Troubleshooting Guide