Case 1845B-Lost all hydraulics-where do I start looking?

Using the skid steer and attempted to perform an 180 degree turn in some soft dirt. Lost all power to wheels immediately and bucket in 15 seconds. Engine running fine. No noises. coupler OK. Hoses OK. Fluid level OK. Tried to tow slowly, but could not due to hydraulic resistance. Loosen lines to pumps and towed slowly into garage. I'm a gearhead, but have not done much hydraulic work. Pulled charge pump. Looks OK, shaft OK, poured oil in inlet and turned shaft by hand and it pumped. What do I look for? Is there anything that commonly breaks on these old units? should I pull the main pump and check the shaft? Any ideas are helpful. Thanks

Case 1845B-Lost all hydraulics-where do I start looking?