I use PB Blaster on rusted / frozen stuff, swear by it. Once it is loose, however hose it down with WD-40, or LPS2.

If the truck was sitting for a while, the Master Cylinder and the wheel cylinders may need attention. Brake Fluid is Hydroscopic by nature, absorbing up to 17% by volume of moisture from the air. This moisture turns steel parts to rust. Which then freezes up master cylinders, wheel cylinders, etc. (I learned more about brake fluid, hydraulic fluid and braking systems during a research project than any person would care to know)

I personally would run new lines and not trust my life to a 54 year old brake line or hose.

Just my $0.02

The problems we face today can not be addressed at the same level of intelligence we were at when we created them - Albert Einstein Or with the same level of $ - Me