I don't know a lot about tractors other than I've spent enough time on one to never need to do that again, but I have done quite a bit of work with industrial hydraulics. It was all about pressure. The suction side of the pump only connected to the reservoir and was fairly sensitive to restrictions. A cavitating pump is not a good thing. With an up-acting press, you pumped it up. The weight of the cylinder and platen would return it to the open position after the valve released the pressure on the piston and vented the cylinder back to the tank. With a down-acting press, gravity might help filling the volume if you used a prefill valve, but then you built pressure with the pump. To get the ram back up, you pressurized the underside annular ring of the cylinder and vented the other oil back to the tank. In other words there were single and double acting cylinders, but you were applying pressure, not sucking oil out.