Front brakes locking up

I have '87 Honda Civic sedan auto 1.5 3bl. Let me start with the trouble I've had since I first bought car. When I first bought the car the front brakes would lock after driving only two miles. So I replaced rotors, calipers, and pads, which did NOT help. Then I replaced front brake lines from advise from dealership. That didn't help either. I took it into a shop and had it checked out. They told me they believed it was master cyl. AND booster. Well I replaced those two things and it helped for about 4months. After 4 months brake pedal started getting very soft and when I had to come to a complete stop the motor would die. Scary. Anyway the master cyl. And booster were under warranty so I replaced them both again. Now motor doesn't die but front brakes are locking up again. I've been told there is an adjustment between booster and master cyl. On the shaft of the booster. There is not could shims be used between master cyl. And booster? The brake pedal has an adjusment on it between it and the booster. One repair manual suggested one possible cause could be proportioning valve. Well what do you think? Any help would be greatly appreciated Thanks. Adjusted booster pushrod like described in e-mail didn't help.