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Hi, I think that PORBLEM IS SOLVED!

I drilled with 2 mm drill the PTFE tube where the M4 nuts are holding. there was pretty high friction because the PTFE tube was squeezed under the nut.
Now it's working very nice and seems to be OK. also added a small chamfer to the tube inside, so it's also much less trouble replacing fillement!

I would advice to add this tip to the instructions...