I must've missed the part where it said the car has power brakes.

Here's something to look into.... it might be related.

A few years back, I had a '74 AMC Hornet with Power Brakes. A shotgun mechanic had been at it for the old lady owner, so many thing were done incorrectly or on the cheap. One issue I had with it from Day 1, and took me MONTHS to figure out was this: the brake pedal was mush for about half it's travel, and would accomplish diddly squat. But as soon as you'd hit that halfway point, the pedal would get a little stiffer, but you now had ALL THE BRAKES. Usually front right would lockup first. The car was virtually undriveable like that.

Anyway, after rebuilding all four corners, changing rubber and hard lines.... it still did it. So, that only left two pieces of the puzzle. A new-looking master, and the booster. I wrote off the master since it looked new! Well, the instant I removed the master, I'd found the problem. The pushrod from the power booster came out with the master!! Whoever did the master last, didn't know the difference between a power or manual master and had left the manual style pushrod locking clip in the master. This caused the pushrod to jam inside the master, not sit inside the cup in the booster like it's supposed to!

Well, after fixing that, the car had perfect brakes.

If that's possible to do on your '57, I'd suggest checking that out.