The function of the restrictor for noise reduction is a bit complicated. It is not just the pump that causes a noisy system. Power steering hydraulics are a bit complicated. The pump also has a flow/pressure control mechanism but the real magic is in the rack itself, in particular the rotating valve assembly as part of the pinion gear input. When things are at rest there is little pressure and the fluid just flows through that valve setup. When you turn the wheel the shaft actually twists  and the valve directs fluid to one side of the piston in the rack. Turn the other way and pressurized fluid pushes on the opposite side of the piston. Flow control and back pressure is used to prevent the system from over reacting and turning the wheel faster than you want. That restrictor helps to dampen the interaction of those valves both for noise and smooth comfortable operation. 

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of John Rodgers
Sent: Friday, January 15, 2016 9:43 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Power steering

Can anyone explain to me what the restrictor in the high pressure hose from hydraulic pump to metal line does, and how it works to do its job?

Looking for enlightenment!

John