rear brake upgrade on 70 dart

I would try NAPA for the rear brake parts. They are a pretty good supplier and the price is not bad. I would also replace the front calipers, brakes and rotors. Then bleed the brakes until you get new fluid coming out. This way you know everything is new at the brakes. Any other issues that come up will have to be in the brake lines, master cylinder or the brake booster. The cost is not to bad to do this. It is better to be safe then sorry with these old cars.