My '57 3100 shortbed has 3/16" going to all the wheels & going to the "T" at the rear axle. Probably same lines at the master too, but I didn't crawl under the truck. Of course I can't swear that's how it left the factory or that it wasn't changed by the PO. 3 years ago I bought F&R wheel cylinders from flaps and they fit fine. I guess I am wondering why you are worried about line diameter as long as you get the right thread into your wheel cylinders & hoses. You could also call some of the usual venders and see what diameter theirs are. I'd think 1/4" to be pretty darn big for a 1/2 or even 3/4 ton.
