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Once bled, leave the pads out and clean and dry everything. Wrap a spacer block in paper, and fit it into the caliper.
Apply pressure at the brake lever using a toe strap or similar, and leave for a few hours (overnight if you have time).
Once enough time has elapsed, undo the strap and give the brake a test squeeze. Check the paper for any evidence of fluid leaking out from the pistons.(Apols if egg sucking 101)
Have you done any pressure tests after bleeding?