My hunch was right, the master cylinder did fail. After I got the first version of this swap done I exchanged messages on instagram with a guy who has a LS-swapped C10 that is running a T56. Coincidentally, he explained to me that he has had several Tilton master cylinders fail on him. I mentioned that I hadn't had any trouble with mine, but it appears my luck didn't hold forever. I'm now running a Wilwood 260-15098 master cylinder. It is a 3/4" bore, just like the Tilton I was running, but the pedal pressure feels better than I remember the Tilton ever feeling. I'll update here if the master gives me any trouble. I may update the parts list of the main thread to reflect this if I can find the time to invest in editing it.