This is a great explanation about the limitations of vacuum-driven system and the advantages of hydraulic ram pumps! Your observation regarding the vacuum reservoir is correct. Raising the output level above the reservoir would disrupt the system.
A hydraulic ram-pump, on the other hand, is powered by water flow and relies on constant pressure. The main difference is in the way energy is harnessed. The hydraulic ram pump relies on pressure surges generated by
hydraulic valves in order to push a fractional amount of water uphill. These valves control the water flow and create a water hammer effect that allows the pump to work efficiently without external power.