Two Line System - Raymarine ST5000+ Owner's Handbook Manual

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If the vessel has two steering positions, check valves will already be
installed to ensure independent operation of the two wheels.
Minimisation of hydraulic fluid loss during connection of the drive
unit will help to reduce the time and effort required later to bleed the
system of trapped air.
Absolute cleanliness is important since even the smallest particle of
foreign matter could interfere with the correct function of the steering
system check valves.

Two line system

A typical two line steering system is shown below. Hydraulic fluid can
be pumped into the ram in either direction, depending on the direction
of helm pump rotation.
The autopilot pump is connected to the system as shown below.
ST5000 Plus Sterndrive & Hydraulic Autopilots Owner's Handbook
Note:
The illustrated system
has a check valve located
within the helm pump.