Combined load-sensing proportion and relay valve for an air brake system

A combined load sensing proportion and relay valve assembly for an air brake system comprising:

• means defining a flow path through said valve assembly between an inlet port and an outlet port, said flow path including an outlet pressure chamber;
• a main valve mechanism including a valve seat and a valve disc, said valve disc being biased into engagement with said valve seat to block said flow path;
• a first pressure-sensitive piston including means projecting from one side thereof for abutment with said valve disc to unseat said valve disc from said valve seat to effect opening of said flow path through said valve assembly;
• a second pressure-sensitive piston;
• engaging means extending between said first and second pistons to enable free relative movement therebetween within a limited range and causing said second piston to abut said first piston in opposition to movement of said first piston to unseat said valve disc when said pistons move relative to each other beyond said limited range;
• control-pressure chamber means having a control pressure therein acting against one side of said second piston to urge said second piston to move away from abutment through said engaging means with said first piston;
• stopper means for limiting the extent of movement of said second piston in response to said control pressure in said control pressure chamber means;
• indicated-pressure chamber means indicated pressure created therein upon actuation of said braking system, said indicated-pressure chamber means being configured to cause said indicated-pressure therein to act simultaneously against both said first and said second pistons, said indicated-pressure acting upon one side of said first piston to drive said first piston in a direction toward unseating of said valve disc and against one side of said second piston in opposition to said control pressure;
• and means for applying the pressure in said output pressure chamber against the side of said second piston opposite the side against which said indicated pressure is applied, with the pressure in said output pressure chamber being balanced against said indicated-pressure to open and close said valve assembly flow path in accordance with the control pressure developed in said control pressure chamber means.