There seems to be a bunch of confusion here.
For me, the master cylinder 'front' is closest to where the plunger enters it.
There are some differences between master cylinder resevoirs, mainly for drum/disc masters. That is because disc calipers are larger and need more fluid, so the disc resevoir is larger than the drum resevoir, even though the drums will often will have larger diameter brake lines.
Disc brakes need more pressure than drum brakes. There is more pressure from smaller diameter bore master cylinders- ie 1" is more pressure and 1 1/2" is more volume. The bore is the principal difference between drum/drum, drum/disc and disc/disc master cylinders.
Someone mentioned the front brakes being applied a little after the back brakes by the master cylinder. That isn't true. The combination valve or metering valve/stand-off valve does that and it is external to the master cylinder.
If you are putting a drum/drum master in your truck I don't think it matters which half of it goes to the front or back brakes.
You will need a proportioning vavle, metering valve and 2 residual valves in addition to the dual master. Your best bet would be a combination valve and residual valves that are designed for a drum/drum set up. It does matter how you hook up the combination valve, since one outlet is for the rear and one is for the front. If you use individual valves rather than a combination valve you would put the metering valve in the front brake circuit, the proportioning valve in the rear brake circuit and residual valves in both circuits. The residual valves are only necesarry if the master cylinder is lower than the brakes cylinders, or may be lower than the brake cylinders if you are going up or down a hill, which is the case with these trucks when you put the master in the stock location...
MP Brakes is a great company and will have exactly what you need. They also have a good tech line and you can call them with your questions and report back if you want.
