Brake Problem

[swb70]

I have a 1970 chevy pickup and the rear brakes lockup on me when I hit them.
I have to bleed them off to release them. Was told that the problem may be the porportional(spelling wrong) valve. I have never had to replace one of these but guess the can go bad. All the brakes were rebuilt about 5 yrs ago.
Shoes, wheel cylinders, hoses, master cylinder. Could this be the problem. Already tried to buy one at local auto parts and was told that this was a dealer part. Any help will be appreciated.. Thanks Ben