I know of a welding equipment company that, back when these were new, eliminated the inch function on all their fleet of 50+ c500 Clarks (not because of leaking brake fluid into the trans, but because they didn't want that function in their operation), and the trucks lasted a LOT longer than most C500s in daily 24/7350 operation.
Depending on the size of your truck, you may have a 2 master cylinder setup, and you MAY just have trans clutches that have been contaminated by brake fluid leaking from the left master cylinder reservoir, into the transmission, through the inching circuit.