After we had the tires on the truck, but before we took it off of jackstands, we decided to run it a bit to get the new fluids well circulated and to check for leaks. Upon stopping, we went to put it in Park, and were met with a horrible grinding noise. Apparently, the rear brakes weren't working. After some head-scratching and re-inspecting the rear shoes and drums, we noticed a leak from the master cylinder between the plastic reservoir and metal base. After shopping around a bit at local parts stores, we found a better deal with no core charge at Summit, and we had the new master cylinder (p/n DHB-M39646) in hand two days later; the mystery of why the front brakes were gone but the rear brakes were great was solved.