Hey Jim,
* It’s been a while since I was elbow deep in that project. But I seem to remember the flare heads on the lines are different sizes…and the inputs on the Shuttle valve(PDWL) front-back are different thread sizes. I think this is to prevent you from installing it backwards if you remove it. But no guarantees if you’re running your own lines and putting your flare heads on yourself.
* If you have the reservoire that looks like mine(squarish), then the line closest to the Booster/Firewall is the largest volume one, and should be plumbed to the front brakes.
* Get yourself some bleeder screws with a ball valve in them, or do the trick with some vacuum hose and a little throw-away clear container of clean brake fluid, so it doesn’t suck air in when you bleed the brakes. You might have to set it up and open the bleeder screw and be over by the shuttle valve to force it one way or another. The valve should move pretty easy back and forth. Just take the bottom access port off and use a flat screwdriver to push it forward or back - to open each circuit. Or just hold it centered as someone in the car floors the brake pedal.
* Once open, there should be nothing stopping the flow. As long as you don’t suck air bubbles in, you should be able to flush the lines out with clean fluid. If you get pressure out of the lines, you can rule out the MC. Holding the shuttle valve centered should rule that out. The only thing left would be the wheel cylinders themselves.
* But I’ll tell ya - if there’s even a little air in the line, those shuttle valves will do their job and shut that side off. And no amount of pushing will let fluid go to that side. That’s why people get frustrated with them and remove them. But they are actually performing their job well! That’s exactly what you want to happen on the road with a leak! I remember spending 2 weekends convinced something was broken. It was just that shuttle valve returning to the closed position when I tried to bleed the rear wheels. It’s almost like you have to have 3 people - one to pump the pedal, one to open the bleeder screw and close it immediately, and one to hold the shuttle valve centered. You can actually get by leaving the bleeder screw open slightly if you have the vacuum hose into clean fluid setup, because it can’t suck back much. So that means you can be on the shuttle valve, and back to only needing 2 people. Once you get “most” of the air out of the system, the shuttle valve won’t be so finicky, and you can go back over to each bleeder screw and be quicker about closing it off for a final flush.
* Long way to describe it, but I hope this helps somebody.