automarc has described a piston pump within a reservoir. However in those old dumps you'd only find gear pumps, a cast iron setup with sliding or rotary valve. No reservoir was used and the oil contained within the systwem was simply recirculated unless it had other hydraulic equipment as on a highway dep't unit. As for the PTO's they are extremely reliable but are made for specific transmission models so it is important that you find one from the same tranny as yours. If you use a wrecking yard hoist replace all the hoses and fittings for safety reasons. Inspect the cylinder rod for pitting.
The PTO's are mainly from Chelsea, Muncie and Tulsa. The gears, shafts, shift forks, bearings/bushings as well as seals all go bad so inspect it carefully.
