Use an air compressor and a blow gun with a rubber cone-shaped fitting to blow air into the caliper body through the hole where the banjo bolt went. It doesn't take much pressure. Use something to absorb the energy from the first piston that flies out (I use wood door shims or rags jammed inside the caliper body - like a champagne cork poppng out, the pistons can carry more energy than you think). Once the first piston is out, hold it in place over its seals to repeat the process for the remaining pistons.