Here's a diagram of front to back brake lines

[davepl]

This is for the 72 with discs. Thought it might come in handy for someone. I also added a bunch of details I've had to dig up along the journey. If someone spots anything wrong, please let me know!

On 1/2 ton trucks the front reservoir on the master cylinder controls the front brakes.
One 3/4 and 1-ton trucks the rear reservoir on the master cylinder controls the front brakes (excl vacuum over hydraulic on 1-ton)

Based on my asking and looking, it appears that on the 3/4 ton the line that:

- The rear reservoir port is routed to the front port of the combination valve
- That front port of the combination valve will come out the front-bottom; this is the FRONT brakes
- The rear port of the combination valve is the REAR brakes
- The OD of the front port tubing (front brakes) is 3/16"
- The OD of the rear port tubing (rear brakes) is 1/4"

I got these last two reversed, and discovered that it's pretty much impossible to find a tube nut that will hold 1/4" line and fit that tiny port. So I drilled out the nut to just over 1/4" (just enough for the pipe to barely go through the nut) and the nut was paper-thin. So the front port on the combo valve has to be 3/16"

To reiterate: front brakes 3/16", rear brakes 1/4" (except the crossover pipe on the rear axle, I think that's 3/16" but don't recall offhand).