External Filters

It might struggle to Prime if there’s water in one of the pipes.

Just to clarify, all the priming button does is start a siphon, and it’s hard to start a siphon if there’s already water in the pipework.
When I clean a filter I always do it during a waterchange, so the return pipe is easily emptied because it’s above the water level. I put a tap on the intake pipe near the filter, and close the tap while I’m cleaning the filter. Then when it’s all assembled again with the pipes reconnected, I just open the tap and the filter fills, because the intake pipe was full of water.
Hope that makes sense.