If you fit a pump to your steering system and the wheel spins when the pump is driven, then you need one of these valve blocks to stop the oil back feeding to the helm wheel.

Marine Autopilot Pilot Operated Check Valve

The marine autopilot pilot operated check valve is a line mounted valve that is used to lock steering cylinders. The G1/4 valve can also be used when a helm pump is without check valves or to prevent the autopilot pump back driving the helm wheel.
 
Description
 
This G1/4 (BSPP) in-line check valve is designed to close the ‘C’ ports until a pilot pressure is applied to the ’P’ ports in order to move the check valve piston and so open the port. The body is made from anodized aluminium for protection against the harsh marine environment whilst the internal components are made from hardened and toughened steel for extended life.
 
Application
 
Designed to be used to positively lock steering cylinders or to prevent the back drive of the helm steering wheel by the autopilot pump should the helm pump not have check valves.
 
It is designed for use with both balanced and unbalanced cylinders. Where an unbalanced cylinder is used the additional ‘R’ port is connected to the helm pump reservoir bottom port.
 
Features

• Positive locking of the cylinder.
• Hardened check piston.
• Toughened check seats.
• High grade chromium balls.
• Suitable for balanced and unbalanced cylinders.
• Mounting holes.
• Port identification.
• Anodised aluminium

 
Technical Data
 
Rated flow                       4.5 l/min
Maximum pressure         72 bar
Pilot ratio                        2.25:1
Port size                         G1/4 (BSPP)
Ambient operating
Temperature                   -15 to + 55 deg C                    
Fluid                                ISO VG10 to VG40 hydraulic
                                       mineral fluid to ISO 6743-4 HV                                 
Weight 0.31 kgs