A proportioning valve is a valve that relies on the statics to supply a reduced pressure to an output line.
A simple example is where spring load applies a reducing force so that the output pressure is reduced. Proportioning valves are frequently used in cars to reduce the brake fluid pressure to the rear brakes.
- Fundamentals of Automotive Technology: Principles and Practice. Jones & Bartlett Publishers, Aug 1, 2013 pg. 1013