Your brake lights were on all the time, now they only work when the parking lights or headlights are on. The good news is that your brake light switch is working and your wiring to the brake lights is good. So something you changed made a difference. Find the terminal on the headlight control switch that sends power to the brake light switch without having having the parking or headlights on. Since you know the rest of the circuit is working that should give you proper function of the brake lights.
I see that your truck is a second series '55 and I don't know if it's wired the same as my '48. On mine the terminal adjacent to the brake light terminal is the one for the dome light.