When you bleed the brakes, the bleed nipple must be the highest point on the caliper or air will remain inside the caliper. You will also need to ensure that there is no air in thr loop between the calipers.

Have never used clamps so cannot comment on this method. You'll need to pump the brake lever on fresh oil to build the required back pressure for the cylinder to operate correctly. This can take some time but eventually the prressure will come.