Combined load sensing pressure proportioning relay valve

A combined load-sensing and pressure-proportioning relay valve, for a vehicle air brake system, comprising, in combination, a relay valve unit including a relay valve body formed with a first inlet port for connection to a source of indicated pressure for controlling brake application, a second inlet port for connection to a source of brake supply pressure, an outlet port for connection to the air brake actuators, and an exhaust port, a main valve disc in said relay valve body controlling communication between said second inlet port and said outlet port and having an axial exhaust bore communicating with said exhaust port, means in said relay valve body biasing said main valve disc to a closed position blocking communication between said second inlet port and said outlet port and said exhaust bore, and a control piston assembly in said relay valve body operable to adjustably displace said main valve disc to an open position;

• and a modulated-pressure control valve unit communicating with said relay valve unit and supplying, to said control piston assembly, a pressure modulated according to the loading of the vehicle;
• said control piston assembly including first, second and third pressure-responsive pistons;
• said first and second pressure-responsive pistons conjointly defining, with said relay valve body, an indicated pressure chamber communicating with said first inlet port;
• said first pressure-responsive piston and said relay valve body defining an output pressure chamber communicating with said outlet port;
• said third pressure-responsive piston and said relay valve body defining a modulated-pressure chamber;
• said first pressure-responsive piston being engageable with said main valve disc, responsive to the indicated pressure, to close said axial exhaust bore and to displace said main valve disc to an open position connecting said inlet port to said output pressure chamber;
• means biasing said first pressure-responsive piston to a position disengaged from said main valve disc;
• a loading spring biasing said second pressure-responsive piston toward aid first pressure-responsive piston in opposition to the indicated pressure;
• said modulated-pressure control valve unit communicating with said second inlet port and said modulated-pressure chamber;
• means biasing said modulated-pressure control valve unit to a closed position blocking communication between said second inlet port and said modulated-pressure chamber;
• a stroke adjuster operatively associated with said modulated-pressure control valve unit and operable, responsive to vehicle loading, to open said modulated-pressure control valve unit;
• and a normally closed valve in said modulated-pressure control valve unit opened, responsive to the modulated pressure exceeding the brake supply pressure upon a decrease in the brake supply pressure, to equalize the modulated pressure and the brake supply pressure.