Jimbo....
One diagnosis technique that I haven't seen mentioned is cracking the caliper bleed screw when the "pressure buildup" / excessive brake drag symptoms are observed. It you get a spurt of fluid and the brake releases, then you know you have some sort of issue with the hydraulics....either a brake line interior has failed and is acting like a check valve, the recouperation hole in the M/C isn't functioning, or the brake lever is incorrect masking the recoup hole.
If when the brake drags, cracking the bleed screw doesn't help, then you must have a mechanical drag....either from a sticking piston, sticking pad or binding caliper arm pivot.
Good luck in your troubleshooting.