Re: Wiring Master Vol. to 2 P-Rails, 2 Vol., 2 Tone, 2 DPDT Switches, 3-Way PU-Select
If the pickups connect to the pots first, and then the switch (as I am guessing they do, because that's what they do in a typical 2V/2T setup), then your master volume will go between the switch output and the jack.
In other words, ya see the wire that runs from the switch to the jack? That gets cut, and those freshly cut ends both connect to the same lug on the pot. Same signal path from switch to jack, but you have now installed a controlled leak along the path, A.K.A. a volume knob. When your volume knob is on 10, the leak is sealed, and everything goes to the jack. As you turn the volume knob down, more and more signal leaks out, and less makes it to the jack.
The ground wire must connect to the master volume pot as well, and then to the ground lug on the jack (i.e. the one that isn't the hot lug). You also must wire the ground lug of the pot to the pot casing. This can be done by bending the lug, or by using a short wire. I prefer using the short wire, in order to keep pots as unmolested and re-purposable as possible. In fact, I usually just strip back extra insulation off of the ground wire, so the exposed end is long enough to reach the pot casing).
Using that many controlled leaks (i.e. volume and tone pots), I would tend to favor higher value pots in order to retain high end. Pots leak a little bit, even when you have them set on full. The more pots in the circuit, the more potential for all those little leaks to add up to something audible. I would use at least 500K pots for the individual volume and tone controls, and probably a 1M for the master (but at least 500K).
