No, although it can make it easier to keep the oil from foaming on the initial fill and run. It is simplest to think of a hydraulic helm power steering system this way:
1)There are 2 main hydraulic circuits
a) The supply circuit, reservoir > pump > helm > cooler > reservoir, which is always circulating oil (well, except at steering lock!). This circuit can self bleed because the air pushes back to the reservoir. A larger reservoir "can" help this self bleed because you "may" have enough fluid on initial startup to avoid running the reservoir dry and pump air back into the system and foaming the oil. If the system is filled and bled properly, a larger reservoir is not necessary unless your steering system geometry is affected by trim angle (twin ram, twin drive, one drive up, the other down, not recommended, will usually break one of the plastic thrust pads off when it enters the gimbal ring!).
b) The steering circuit, Port(L) < helm > (R)Stbd, which only moves oil when the helm turns.
2) Now, when you turn the wheel to stbd, ports change in the helm to allow feed from the supply circuit (pump) to route to the steering circuit through the g-rotor in the helm to the stbd(R) line and to the appropriate cylinder port(s). When the drive starts turning, oil is returned from the other cylinder port(s) to the port(L) line at the helm. This oil is routed through the helm to the reservoir(T) port, through the oil cooler and back to the reservoir.
3) If there is air in the steering circuit, some will probably get back to the helm and get pushed back to the reservoir to burp out. The problem is that on most boats, there is enough line length and cylinder volume that you will probably just have an air "slug" moving back and forth somewhere in each right and left steering line/cylinder. This slug of air is what I said "compresses" when the pump is running, lowering the fluid level, which then rises again when the pump stops turning. Without getting too much more technical, this happens between the 2 circuits because there is some slight leakage between the 2 circuits in any helm.
4) What most bleeding procedures try to accomplish is pushing oil out each steering line until there is solid oil. The Merc ITS does this through little brake bleeder style valves on the steering line connections. These bleeders are inside the boat and you can connect a line to them and run them back to the reservoir, no lost fluid, very little mess. Some of the other manufacturers have you turn the wheel (engine off) until the cylinder is bottomed, remove the external line connection closest to the piston, then turn the wheel slowly in the opposite direction until you have solid oil coming out that line, watching the reservoir level. Reconnect the line. Repeat for the other lines and steering direction. Because the cylinder is bottomed, very little air can be in the cylinder on that end of the cylinder. When you get solid oil in the line, most of the air is removed, for that line/cylinder direction.
5) Helm displacement size: The amount of fluid displacement per revolution. This determines the number of turns lock to lock for a given steering cylinder setup. Most systems are setup for 3-4 turns lock-lock.
6) Orifice Size
This controls pump flow, mainly improving things at slow pump speeds. It can only help so much, don't go too far. RPM helps more (pulley size). Remember how the steering circuit connects to the pump output?. If there isn't enough pump pressure/flow, it will only let you turn the wheel as fast as the pump can supply fluid (well, it does switch to "manual bypass" in the helm, but it's so hard to turn, it feels like you hit a steering input limit). If engine speed drastically helps steering at the dock, you have a supply problem. Check the filter, if there is one!
Now that I have written a book on the subject, let me know what you find.
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