Suspension rebuild and disc brake swap...by a noob for noobs

Before putting the suspension all back together I replaced the master cylinder. The most obvious difference in the disc/drum MC is that the disc reservoir is larger because disc brakes require more fluid flow than drum brakes. Less obvious is that the MC ports for drum brakes have a residual pressure valve, but the disc ports do not. (Some say later drum brakes don't have them either, but they were there in 1974. Then there's a question about whether modern replacement wheel cylinders require the ports to be there, because they may have mechanisms in the cylinders that make the residual valves unnecessary. To be on the safe side, I chose to use all factory replacement parts.)

My new master cylinder came with a new rubber ring for the push rod (to keep it from rattling?). The existing rubber was brittle, so I cut it off and installed the new one. I bench bled the new M/C according to the included directions and mounted it on the firewall. Since the brake lines were still disconnected at the wheels, I left the plastic plugs in the M/C output ports to prevent fluid draining out. That same day, I applied PB blaster to the line nuts on the old distribution block (directly below the M/C, mounted onto the frame).

Next day I removed the old distribution block and installed the new proportioning valve. (The proportioning valve is necessary with disc/drum brakes because with equal line pressure the drums would grab more than the discs. The valve reduces the pressure to the rear lines, which balances out the braking force. You can partly achieve that by using smaller rear wheel cylinders, but the proportioning valve is more sophisticated because it provides full rear pressure sometimes and reduced pressure only when necessary. The drum/drum distribution block always sends equal pressure to front and rear.) Because the input lines were already disconnected from the M/C, it was easiest to leave the input lines in the old block until after it was out of the car, so the lines came out with the block. Before installing the new proportioning valve, I went ahead and connected the input lines to it. Because the new valve's ports are not exactly in the same places as the old ones, I had to bend the lines a little by hand to make them reach from the new valve to the M/C, and I had to slightly bend the other existing lines as well. My lines were in very good condition, but if they had been rusty at all I would have replaced them all with NiCopp or prebent lines.

When I connected the pressure warning switch to the new valve, the pigtail was loose, so I squeezed it with pliers until it fit snugly.

I didn't realize it until later, but my new proportioning valve had a leak, as described in this thread. If I do this again, I'll put the valve in a vice and really crank down on the hex before installing the valve.