I have a 68 impala convertible. Stock factory power drum brake car. My car sat many years before mechanical restoration. My question is has anyone come across power brakes that are super sensitive. You barely touch the brake pedal and all wheels lock up. If you drive the car carefully no problem but apply brakes quickly and it results in four wheel lock up. Keep in mind everything is new or rebuilt. New lines master booster wheel cylinders shoes hardware etc. Any advice as to what to check is appreciated.