Peter
If you allow water to flow back into the filter via the outlet pipe it fills a section within the top tray - see attached picture
if this sections become full of water whilst primming is a problem and you have encounted the problem.
Before priming the system lift up the outgoing water pipe so it above the water level then start priming, once the filter is full, switch the filter unit on - still with the out going water pipe above the water level, once the water starts to flow out of the out going pipe then ease it back into the tank water.
I do the above when primming my 2080 and since adopting this method - never had a problem since.
Regards
paul