Hi Charles, you need to open the filling loop.
This is best explained in the video here, please let me know if you have difficulty following the video or have any follow up questions.
If the pressure gauge is low down (zero) this would suggest you have a significant leak, as such once you open the pressure valve as soon as you see it get to 1 bar turn it off and check all the radiators for a leak.
I havent heard back from you please let me know if you have any follow on questions.
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