1966 Mustang help with the brakes

You should never try to lock the brakes when stopping. If the brakes do lock, you will slide, you will not stop as fast, and you will have zero control of the car. If your brakes are locking up early, it could be that the wheel cylinders are leaking fluid onto the shoes. If it's pulling to one side, the drums may not be adjusted correctly.

That line might go to the rear brakes. First thing I'd try is jack up the rear, have someone push the brake pedal, and then try to turn the wheel. If it turns, then you probably don't have fluid in the rear, or the shoes are totally worn out. Also, it wouldn't hurt to pull the drums off and take a look inside the brakes. There should be plenty of friction material on the shoes, and the springs should be intact. The adjuster wheel on the bottom should be able to turn fairly easily, too.

How does the pedal feel? It should be quite firm and without much give. If it feels spongy, then there's air in the lines or a seal is bad somewhere and is leaking fluid out.