Brake lines may be part of the problem, but to see if it is pull them, feed a wire into them to see if there is blockage. If there is none, then that's not the problem, not with just a couple of strokes to the pedal to have them lock up. The master cylinder has bleed back holes under the lines coming from the reservoir.
The bleed back hole allows the brakes to release.
Often on older master cylinders the bleed back hole becomes clogged with crystallized DOT 3.
It’s best to pull the master cylinder and remove the pistons before sticking anything down the holes so you don’t damage the seals.
Bill