something in your brake system is restricting the reverse flow of brake fluid.
if all 4 of your brakes are locking up, then as you said, I would rule out restricted or defective rubber hoses at first blush.
I would change out the master cylinder and start from there, if that does not fix the problem then rebuild all wheel cylinders and replace all rubber brake lines.
flush all old brake fluid,
at least that would be my plan, if these item replaced or repaired does not fix the problem, then it would be off to the mech shop
as you must have good dependable brakes to drive the jeep on the highway.