Abstract: A vehicle load controlled fluid brake pressure regulator having a relay valve portion that includes a variable area differential relay valve piston, the effective area of which varies in accordance with different load conditions. A control portion of the brake pressure regulator includes a stepped control piston having two piston faces subject to the load dependent pressure of separate air springs of the vehicle, so as to be axially displaceable for different load conditions. Oppositely inclined control surfaces of two cam members carried by the control piston translate the axial position of the control piston to the relay valve piston via an actuator stem. One cam member is axially adjustable relative to the other, so as to cause the hollow or low point of the cam profile formed by the inclined control surfaces to engage the actuator stem for any desired "empty" load setting of the relay valve portion.