Got and issue with my NSR brake.....

When I press the brake pedal, there is a creaking noise similar to the noise given by top mounts or bad steering motor. I can feel a slight stiffness through the pedal at the same time as I hear the creak, this does not happen with handbrake applied and is coming only from NSR brake.

I stripped, cleaned & re-greased everything inside both back drums last weekend hoping that would cure it but no joy. Everything inside back drums was covered in dust but generally corrosion free and all looks to be in good order. No binding and brake performance is as good as I would expect it to be but I would like to try to narrow this down and sort it. Anyone got any ideas?

Thanks in advance

Ben