Hi Ignacio,
As I mentioned in my email response, you need to use the CORRECT tubing. Which is 2.5mm ID 4.5mm OD as specfied on the Hardware page here: https://chi.bio/hardware/
Then, if you are still having problems, you could add tape to the pump head to make the seal tighter.
Ensure you have fully read the operation manual since it discusses each of these points (e.g. in the setup parts and also troubleshooting guide).
Harrison
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