As per the above advice. But I'd be inclined to take out the pads, then ease out the pistons a little further, clean them off with warm soapy water (car shampoo, not washing up liquid!) And use some fine, 0000, wire wool around the pistons to clean of the crud. Only then push them back in. You'll want to presumably give the calipers the same treatment.
Bleeding the servo brakes is not quite as straightforward as normal brakes but it's not difficult either. Lots on here about got to do it. In short, you have a circuit from the lever to the servo pump that needs bleeding. You then have the 'power' circuit from the pump that actually pushes the pistons. When doing the latter, you can use the pump itself to force through the new fluid.
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