Are you using dual valve quick disconnects on the GLA filter? I find they are useful for re-priming. I put the disconnect close to the filter on the inlet side. And the other, close to the top of the pump.
When I'm done with maintenance,
1) reconnect the inlet side disconnect with the valves closed,
2) leave the disconnect apart on the outlet, with the valve open,
3) slowly open the valves on the inlet side, which begins to siphon water out of the tank, into the filter (air will be escaping from the disconnect on the outlet side of the filter),
4) pay careful attention as you begin to hear water and see it coming through the pump, and then quickly close those valves,
5) reconnect the disconnect on the outlet side, open valves and plug in the filter.
I can get this filter going without spilling a drop. It is a million times better than the Oase I had, or any Eheim I've owned.
Not sure if this makes sense. I could make a video if you would like. Filter maintenance is today

--Steve