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Squeeze both tabs fully, pull away from radiator. O-ring just gets sticky; assist with prying tool if required.
I always use a set of wide jaw Multi grips, the ones that have several grooves in them to allow you widen the jaw opening when they are fully closed. This allows you to place the flats of the jaws perfectly on the tabs squarely. Compress tabs and lever/tap off.
Order yourself anew radiator now to save time. Once the cooling jackets start to leak its a sign that the radiator is soon on its way out.