If you have got to the end of the adjustment, it might be worth removing the rear brake drums and checking the amount of lining thickness left on the brake shoes.
Might not be the issue, I've fitted new shoes, and slave cylinders, tried to adjust handbrake and I've run out of adjustment too. Finally managed to set it so the car passed MOT, but after driving about 50 miles handbrake was weak again. So far I haven't been able to find solution, and with that it won't pass its next one