What size truck?
I don't have a 1/2 ton but I think they may have used some 3/16"?
On my larger trucks they are all 1/4" front to back, no 3/16".
Typically drum brakes on a pickup truck use 1/4". In some cases a truck with small wheel cylinders use 3/16"
Typically disc brakes on a pickup use 3/16".
Sometimes 1/4" is used going to the rear axle where it then splits to two 3/16" lines, either disc or small drums.
The real answer is you need enough fluid flow to actuate the brakes quickly. On the other hand oversize lines contribute slightly to a spongy brake feel.
Simple answer is to use what the factory used for the original vehicle if you have the original brakes. Or if you have unoriginal brakes you should use the size lines that the factory used on whatever vehicle the brakes came from.
Grigg
