you don't mention a serial number series of this truck, and I wonder what made you decide to service the transmission in the first place, also, if the trans is overfull, there are a couple of places that can happen,,, on some clarks, (c500s) the inching slave cylinder is on the side of the transmission, and if it leaks internally, the brake fluid goes into the transmission, which eventually will soften all the gaskets, clutch material, and seals, and ruin the transmission because of the leaks and faster wear on the clutch plates. this clark has 2 master cylinders, and only the left master cylinder drains into the trans.
you may only have a stuck inching spool or linkage.