You should be fine with 3/16" for everything from front to rear, all of them.
I doubt the rear axle lines are 1/8", probably 3/16", but I don't usually play with small trucks.. so you're probably right.
If I were to deviate any from all 3/16" lines it would be 1/4" line from MC to proportioning valve and from valve to the T at the rear axle where it splits to 3/16. But if you have a relatively small axle with the appropriate brakes the 3/16 should be fine for that.
In my case I have relatively large disc brakes front and rear, and the factory setup was 1/4" feeding the rear brakes where it split to 3/16" on the axle.
Some folks will say disc brake systems use 3/16" for everything, (I believed that at one time) and some systems do, but not all. The disc I have from the P30 chassis uses the 1/4" rear line, and the last project truck I had used 1/4" for every bit of the brake lines, and it had disc front and rear.
In the end you are trying to duplicate a factory system, not redesign one. So what ever size line the wheel cylinder and calipers had originally, then that's what you want to use again.
Grigg
