bleeding heater matrix

Hi guys read every forum haynes ect on how to bleed the coolent system on my l series I believe I have a massive air lock in the heater matrix as it's only getting hot when cars been running a long time I've tried with cap on cap off ect how can I flush the system completely of air so I can move on if it's not that I'm going to have to remove the glove box and check the heater assembly if somone knows how to do this help please as I posted in another thread I did have a single pop the day after I fitted a heat exchanger for svo when I was trying to bleed the system but I've had no coolent loss so can't be a burst pipe no idea what else could cause the issue now completely stuck

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