Kevin, I can tell you that I have pulled and rebuild an SD100 transmission and there are no internal brake parts. I also reconditioned the brakes on an SG60 and it had a piston that applied pressure to wet brake discs that mounted outboard of the differential housing. On that system, as I recall, it had a Mico master cylinder that you filled with an oil, which had a special spec. Why not lift one drive tire at the time by tilting the mast forward on a block and see if one or both sides appear to be dragging? I would then locate the brake line to each side or bleeder to crack it and see if it has pressure without the brake applied. Maybe the valve is bypassing? Possibly the pushrod is too tight??