Need help on using ATB Multi Use Skimmer in HOB mode

[SoloSK71]

I just bought this used from kijiji and am not able to get it to keep primed.

I pre-prime it by using the valve to draw air out from the top of the intake loop, so that the mesh wheel will pump water, and by holding my finger over the air intake tube the skimmer will pump a lot of water through, but as soon as I take my finger off then the water level drops and no bubbles come out into the body of the skimmer.

I have tried searching for instructions but nothing comes up on Google or YouTube, just a coupe of reviews that do not give details.

This is my first experience with a skimmer like this and with a meshwheel, so if there is an arcane trick that I have missed that would really be appreciated.

Charles