Have a 1985 nissan 300zx. The rear left disk brake drags when backing up. I also noticed recently that you can hear it grinding while driving. This is not a constant grinding. The car has 200,000 miles and I had the brake booster and master cylinder changed 2 years when the front brakes locked up. Because of the Cost of OEM parts, the dealer installed a cheaper aftermaket booster and master cylinder on the car. This is when the problem started. I then decided to replace the both rear calipers. Any suggestions would most appreciated.