This is the original thread where I was trying to figure out what was wrong: http://www.reranch.com/reranch/viewtopic.php?t=28702
Let me give you the cliff notes. I've wired 2, 3 pickup guitars using the exact diagram you have. The first guitar always had a weak sounding bridge pickup that I always thought was just the pickup.
When I did another one with the same diagram I had some stray solder ground out the middle pickup. So I temporarily took it out of the circuit and played it as a standard 2 pickup guitar. Sounded GREAT. When I hooked the middle back up to the circuit the bridge sounded weak again.
Well, after a bunch of experimenting I realized that the way the Guitar Electronics wiring diagram is, the middle volume is loading down the bridge volume pot all the time. SO, you'll want to do a couple things, first let me show you my finalized wiring diagram:
Let me break down whats going on here. First, I used a push-pull pot on the middle volume. By wiring the switch AFTER the pot, this takes the load of the potentiometer completely out of the circuit when "off". Doing this completely solved my issue and returned the bridge pickup to sounding nice and full (11 months later!).
Here is the original wiring diagram so you can see the difference of how the push/pull pot is wired:
http://www.pbase.com/dkauer744/image/110039191
To get the middle pickup to be uneffected by the bridge volume pot, you really should use 2 push/pull pots but honestly, the middle pickup sounds ok to me as is.
I hope thats helpful.
Part two
- The Gold P90 firebird had the same issue. Unfortunately my Push/Push pots aren't here yet. I had solved this problem by opening up the Pots for the bridge and middle volumes. I scraped the tracings off the board to turn them into a "reverse no-load" pot. What this does is causes the pot to break connection at "0" and go completely open. This not only takes itself out of the circuit but also solved my little bit of volume bleed (you could still hear the guitar some through the amp with the volume all the way off). This is a pretty cool, easy mod to solve a couple of problems.
Anyways, thats it! The bridge pup on the white firebird (KA mini's) sounds better than ever and the Lollar 50's wound P90's sound just killer.
, would make her perfect! 
