Ummm! Most of the old rigs just have a big lug in the side of intake, with hose to the valve on the firewall. That is a checkvalve. Sometimes this valve has small line inside to speedo gear changer, and a gauge if you have, some don't. This line runs down under cab to metal lines back to the rear. Many times those metal lines are rusted out and will leak, hoses rot too. This is special hose that won't collapse when under vac, don't use heater hose. Napa and others should have that big hose. I'm pretty sure the Assembly Manuals have the routing, maybe you need to look under diff headings?
I would think you could come off your V-8 at manifold fitting on back, behind carb, where vac hose to power booster usually is on more modern trucks. Should be a bigger 1/2 to 3/4" pipe fitting with plug maybe, or has a brass elbow with hose barb plugged off? Run it to firewall fitting first, then try all the switches and listen for leaks before deciding what to fix. If it leaks bad, engine will run rough too. Sometimes you need to tap that firewall valve with hammer to knock it loose, or remove and loosen it. There is basically a spring and ball in them, sometimes rust. Don't loose the parts if you take off.
Many times the old hydra vacs get rusty in the master cylinder on them and won't work. Or, the diaphragm is leather and rots/leaks. There are no kits to repair, usually just replace with something newer from junkyard. IF you can find one not too junky. Shop manuals have full repair procedures on them, but you probly won't find parts as mentioned. There are rebuilders of them in CA and elsewhere, but everything I ever sent that was old was too far gone to fix. Rebuild's usually around $300. They also have newer units to replace yours for a little more $. Then the wheel cylinders will probly be bad and leak, the lines all over truck, etc. This will not be a quick and cheap fix, keep that in mind. If any shoes have fluid or rear grease on them, they will also need recover, and new axle seals.
Are You depressed yet? Been there many times trying to get these old rigs rolling again. That's why they are setting around, too costly to fix, just cheaper to replace truck with later model. They run cheap!
