A320 series answer:
Under normal circumstances, with the engines running, normal braking comes from the green hydraulic system. If you have the engines running and positive pressure on the triple indicator with the foot brakes, then the alternate braking system (yellow) or accumulator pressure is showing. That is why you have the break test after engine start. The only other way to have positive pressure on the triple indicator with engines running is to set the parking brake(yellow).
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