Merry Christmas to all

I’m still troubleshooting our cooling system.
Renesis 4 port in Glasair Super II RG

We removed the thermostat, and plugged the whole between the in and out circuit with a threaded plug.
I think I read someone saying there should be a hole in that plug, which I take it was to let air our of the inlet side of the pump.
On a picture of the front housing I think I can see a hole in the top of the pump impeller housing that might perform this function.
Can anyone comment?

Thanks

Steve Izett
Perth Western Australia