these trucks have 2 master cylinders, and the left one is an inching circuit that when the slave cylinder leaks, it is possible that the leaked brake fluid finds it's way into the transmission fluid. At first, this will cause no noticeable problem, and in fact, may help the transmission engage because the effect the brake fluid has is to soften the material the clutch plates are made from. If the leak not repaired and fluid is not flushed right away, it eventually will also soften the seals of the transmission, and over soften the clutch plates, so the trans wears out and slips.
Before I went over the edge with it, I would make sure that the left master cylinder has "freeplay" in the shaft that pushes the spool into the master cylinder, when the pedal is released. I would also see if "bleeding" the pressure from the inching slave cylinder (on the left side of the transmission, not easy to get to) gave you a good inching/clutch engagement.