The hard lines for the brake system should all be 3⁄16-inch. Rubber or stainless flex hoses that connect the calipers to the brake lines should be kept to a minimum length. Front lines are usually in the 12- to 14-inch range and rears for rear disc run 10 to 12 inches. If they get too long, the braking pressure can cause them to expand and you’ll lose some line pressure. I saw a guy one time plumb the whole brake system on his 1934 Ford chassis with stainless flex line – must have cost a small fortune. He said he didn’t know how to bend hardline and this was easier. Well, he could never build enough line pressure to get the brakes to work. So after the car is completed and running, but not stopping, he had to have the brake system plumbed in hardline. You can always do a better job at this when the chassis is going together. Not after the vehicle is complete.