Re: ED Rear Caliper
Are you using the correct tool? There is a rear caliper installation tool used in conjunction with a a square ratchet driver. I bought an ABW branded one for around $50 a few years back.
If you are using the tool and still not getting anywhere its time to rebuild the brakes. Parts shouldn't be too much, new rubber boot for the piston and a few seals. Also a couple types of grease, metho and brake fluid. The factory service manual has quite detailed instructions on the procedure.