Frozen Brakes on '77

[MNiceGuy]

The front brakes are engaged hard on my '77 Scottsdale...to the point I can barely move the truck without starting to break the rear tires loose. Here's what I know so far:

- No power assist
- Front disc / rear drum
- Brake lines (hard and soft) are original to the truck
- Brake dummy light lit on the dash
- Lots and lots of pedal travel. There is some resistance but I can push it to the floor
- No leaks anywhere that I can spot

I've read that I can reset the prop valve which may fix the conditions that are lighting the dummy light but I feel there may be more to it. I've also been told the soft lines may be collapsing from the negative pressure and not letting the fluid back out of the caliper. Is that possible? Where else should I be looking? Could it be a bad/faulty prop valve?

Thanks!