Hi Steve,
A question. When you have to re bleed the brakes, do you get any air out of the lines? If so, you are not fully bleeding them. My mechanic bled the rear first, then the front and then the rears again. If there is still air in the lines then you can jack the rear of the car up and this will force the air when bleeding the brakes, to the highest point which will be at the rear. So the last bleed should be at the rear.
Also, check that the discs are running true. One of my problems with my brakes was that the discs needed machining. Over a long distance, the uneven movement against the rear calipers was causing problems.
Hope you can fix the problem.
George.;35