When using a CNG-high-pressure plunger pump to build pressure, air must constantly be expelled; how can this issue be resolved?

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Hello, experts: I work at a gas station. 1: The issue is that when pressurizing with CNG (using a plunger pump from JX Lakezhou), it’s necessary to keep the release valve next to the return gas open in order to achieve proper pressurization (sometimes even with that valve open, pressurization isn’t possible). 2: This problem occurs whenever we start supplying LNG; it didn’t happen when CNG was used alone. However, once both CNG and LNG started to be used together (sharing the same storage tank), pressurization became difficult, as the return gas had to be continuously released for pressurization to be possible. 3: My own analysis of the cause: (1) I first suspected that it was due to the fact that CNG and LNG shared the same storage tank, but nothing changed even after they were used separately. (During this period, worn parts such as the guide ring and seal ring of the piston pump were replaced.) (2) Is it caused by poor return air flow? (At the moment, it has 5 elbows for returning air.) 4: I would like to ask the experts here for any solutions or better suggestions. Once again, thank you so much to all the experts! ! Waiting online for your reply~! ~! ~! ~!