FYI - Here is the answer I got from Aquacomputer support. (very fast response I might add - nice!)
Thank you for contacting us! If your Ultitube came with a pre-installed D5 NEXT pump,
it must be the "PRO" version of the Ultitube. All PRO version
Ultitubes come with a pressure equalization membrane installed in the top lid
of the reservoir, so will not hold pressure by design.
If you want to pressure check your loop including the reservoir, you can try to
put some plastic foil between glass tube and lid of the reservoir or use this
lid without membrane: https://shop.aquacomputer.de/product_inf…roducts_id=3865
Best regards!
Moldi gets her answer