[bimmer49]

I would be inclined to strip the rears and clean up all of the surfaces. Coppa slip the moving pins and surfaces. Reassemble and REALLY adjust up the auto adjusters with a full brake pedal application and LARGE movements of the handbrake. Get out and check the free movement of the wheel. Pull the handbrake up one click at a time and see which one bites first. Using a wheel brace, try to turn the wheel with each notch on the hand brake.

Assuming there is a load sensing valve fitted in your market....bung a load of weight in the back (100kg) that ought to bring the effort up!