My understanding, and I could be wrong, but I'm quite confident that on a JV33/CJV/JV5 that the dampers should be completely full. I want to think I saw it in some Mimaki document or maybe a video Mimaki has for changing the printhead (could have been for any of the DX5 head models - don't remember) where they state and show opening the valves in the service menu, then install the new damper on the line, hold it upside down, then gently press in the side valve on the damper until ink just starts to leak out trying to make sure that no air remains in the dampers. The older style DX4 dampers should be half full but on the DX5 head machines they need to be full. Vander is def an expert on these so take any tips he might have for sure.
I have run into this on ours before but cannot remember what the problem was.