Abstract: In an air pressure proportioning variable load valve for a vehicle compressed air braking system the operating mechanism for a self-lapping valve assembly includes a fluid pressure responsive piston arrangement which is operable above a given fluid pressure (PI) to adjust the position of the self-lapping valve arrangement relative to a variable area pressure responsive assembly whereby the rising output pressure corresponds initially to rising input port pressure up to the given pressure (P1) followed by progressive adjustment before proportioning by virtue of relative areas of the variable area assembly.