Hello, they are a hit of a pain to bleed, easiest way is to lift front end slightly, turn heaters on, run engine with rad cap off and massage the bottom hose, easier with two people as one can watch for air being expelled and top up header if needed, when you think you have all air out pop expansion bottle cap on and let cool, check levels a good few hours later,make sure the overflow is between the two lines on and header & rad are full,
I'm assuming you have a header tank not seen a hawk bay recently,
If it's the oem subaru rads they have plastic end tanks that crack they are rubbish ! Also check the hoses coming from the header to turbo, sometimes they work loose and you get a bit of leakage