After reading your post and the detailed explanation you provided I was trying to analyze the generic power braking system and determine where in the system a problem as you described might be the cause. The thought that I have is related to the rubber hose brake lines connecting the rigid metal brake with the wheel backing back plates. Aged rubber hose deteriorates internally and can produce the symptoms you have described and experienced. Rather than me explain the various symptoms and resultant effects refer to the attached online link for a complete explanation. If your rubber hoses have deteriorated replacement is not very costly. I hope this helps solve your problem. Jack
https://www.yourmechanic.com/article...ing-brake-hose