Preloading the master cylinder sounds like a bad idea to me. The piston in the master cylinder needs to be fully uncompressed when your foot is off the brake otherwise the port to the reservoir will be blocked off, and believe me it doesn't take much movement of the piston to block off that port. You also won't be able to bleed the brakes when that port is blocked by the piston. The booster to master cylinder rod needs to be adjusted such that there is a small gap between the rod and the piston of the master cylinder. Then adjust the rod length of the other side of the booster to fix the brake light problem.
Also make sure your parking brake is working. It could be that the pistons in the rear caliper is having to travel too far because the parking brake has not been adjusted.
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