It is common to use an early GM power steering pump with a Mustang II power rack and pinion. Most who have tried it, have found that the steering was twitchy or easy to dart.
This valve reduces the fluid flow from three gallons-per-minute to two gallons-per-minute, which cures the problem. For applications using the O-ring style power steering hoses.
Installs in the back side of the pump as shown in photos at lower right.