After testing the two cylinders separately to run on compressed air, I started to test both connected. This took some more efforts in preventing leakages in the connections and the valve bobbins.
Today both cylinders were running, both forward and backwards, on compressed air. They still need too much air, running on 4 bar, but things are improving. See my movie of his on my youtube channel at https://youtu.be/x5nWmz9Z-9k (apologies for the compressor noise).
Next steps will be fixing the lifting links connections, left and right, the reversing screws, and replace the dummy eccentric rods with the real ones.