Last week the radiator went on my subaru sti, so I've had a new one. I looked under the bonnet today and my coolant resovoir is completely empty once again? Could it be because the engine was so starved? I have topped it up and now it seems fine and the coolant level hasn't moved? 

Probably just an air pocket but keep an eye on it.

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Could just be that the coolant system wasn't bled fully after the new rad was fitted . Airlocks are fairly common if care isn't taken when refilling the coolant systems , so it could just be that the air has now worked its way out of the system .

 keep a eye on the levels and temp gauge ,maybe check that all heater matrix and top and bottom rad hoses are getting hot when the fans have cut in . This would check that there's not any other airlocks in the system stopping coolant flow .

Okay, and that doesn't sound too bad? Hopefully it may of sorted itself out now. I will keep an eye on it, thank you for you're input and help guys 🙂