A combined load-sensing proportion and relay valve for an air brake circuit comprising a valve body formed with a supply pressure inlet for connection to a source of air under pressure, an operating pressure outlet for connection to the air brakes, and an indicated pressure inlet for connection to a brake valve;
- a main valve member mounted in said body for axial displacement therein and controlling communication between said supply pressure inlet and said operating pressure outlet, said main valve member being formed with an axial exhaust passage therein;
- means normally biasing said main valve member to a closed state blocking communication between said supply pressure inlet and said operating pressure outlet;
- said body being formed with an outlet pressure chamber communicating with said outlet pressure outlet and with an indicated pressure chamber communicating with said indicated pressure inlet;
- control piston means for opening said main valve member in dependence on the pay load of the vehicle on which the air brake circuit is installed;
- said control piston means including a control piston mounted for axial displacement in said outlet pressure chamber and having, on its underside, a valve seat cooperable with said main valve member, a stepped piston mounted for axial displacement in said indicated pressure chamber and formed with two coaxial portions with respective different diameters, said stepped piston being located substantially coaxially with, and in spaced relation to, said control piston;
- said body being further formed with a regulated indicated pressure chamber defined in part by the upper surface of said control piston;
- said control piston means further including valve means adapted to be opened and closed by axial movement of said stepped piston to control the indicated pressure applied to the upper surface of said control piston and to said stepped piston, and loading compression spring means engaging the upper end of said stepped piston and having its compressive force adjustable in accordance with the pay load of the vehicle;
- said stepped piston being formed with an axial passage therethrough communicating with said indicated pressure inlet through a radial opening into said indicated pressure chamber;
- said valve means comprising a valve seat flanged inwardly of the lower end of said axial passage through said stepped piston;
- an apertured stopper plate interposed between said control piston and said stepped piston and limiting movement of said stepped piston toward said control piston, said stopper plate having an upwardly extending axial protrusion formed with an exhaust valve seat on its upper end and cooperating with said inwardly flanged valve seat to control communication between said indicated pressure chamber and said regulated indicated pressure chamber which latter is located between said control piston and said stepped piston;
- a sub-valve mounted for axial displacement in said axial passage through said stepped piston and cooperable with said inwardly flanged valve seat and said valve seat on the upper end of said protrusion;
- said inwardly flanged valve seat being formed in the larger diameter lower portion of said stepped piston and said radial aperture being formed in the smaller diameter upper portion of said stepped piston and means normally biasing said sub-valve to an open state relative to said inwardly flanged valve seat of said stepped piston.