I'm at my wits end here. Let's start at the beginning....

I've got a 48 1-ton dually. Relined brake shoes all the way around. All new wheel cylinders. All new 3/16" brake lines (I think that's the size... 3/8" fittings on them?) A new master cylinder (NAPA part #NMC M1880) and the ClassicParts 71-984 conversion bracket.

With speed bleeders on each corner I have pushed through at least 5 pints of fluid. I am 99% sure that there is no air in the system. I have adjusted the bakes so that they are "too tight" just to rule out the cylinders needing more travel than I can provide. I still have almost no pedal and the pedal goes all the way to the floor.

The push-rod on the linkage seems to push the master cylinder through it's entire range of motion. At the very bottom of its stroke I can see the bracket flex like it has bottomed out.

The truck does stop but it doesn't "feel" right (or safe.)

My mechanic friend is also scratching his head but is pointing his finger at either the wrong master cylinder for this application or incorrectly sized brake lines. We've checked (and rechecked and rechecked) and there are no leaks or places where air is getting into the system.

Anyone have any thoughts on the master cylinder or the brake line sizes? What else could possibly be wrong?