The is probably an obvious question but I am assuming the pics are work in progress before you added the gpu, and that the other plugged port on the bottom rad will be going to the bottom port on the distro plate, just trying to visualise the final loop. If so then it looks like you are using the top fill port on the Res as the integral return to the Res from the front rad, as well as a fill port. With that type of Res and that loop order you will always be getting air going up the fill line and back into the front rad, it won't get trapped in the Res as it should in normal Res setups. What you've effectively got there is a very small old fashioned Tee line between the Res and the front rad, which were used in the olden times before reservoirs became the norm
It will also be a sod to fill, as you seem to have found. With that tiny effective Tee line you basically have no capacity for trapping air away from the rad, and so you will have to make sure the loop is completely filled, and I mean completely. It would have been better to have used one of the inlet ports on the bottom of the Res, rather than trying to use the fill port as the inlet, if it was me.