Inversion brake valve and system therefor

A valve for use with brake actuators in a fluid-actuated braking system where sources of pressure at varying levels exist at valve ports comprising:

• a valve body having a generally closed cylindrical bore therein, said body having a first inlet port in fluid communication with one end of said bore, a vent port in fluid communication with the opposite end of said bore, second and third inlet ports and a delivery port in fluid communication with said bore;
• a first cylindrically stepped single-piece piston within said bore generally adjacent said vent port, said first piston having a hollow stem in fluid communication at one end with said vent port and defining a first valve seat at its opposite end, said first piston including sealing means circumferentially disposed thereabout to define with said bore a first pressure responsive area in fluid communication with said second inlet port and to define with said bore and said first valve seat a second pressure responsive area said first pressure responsive area being at least as great as said second pressure responsive area but not greater than 1.5 time said second pressure responsive area;
• biasing means contacting said first piston tending to move said first piston toward said first inlet port;
• a second cylindrically stepped piston within said bore generally adjacent said first inlet port, said second piston including a second valve seat facing and generally concentric with said first valve seat, second sealing means circumferentially disposed about said second piston to define with said bore a third pressure responsive area in fluid communication with said first inlet port and to define with said bore and said second valve seat a fourth pressure responsive area in fluid communication with said third inlet port;
• sealing means associated with one of said pistons effective to (i) seal said first and second valve seats to prevent fluid communication between said third inlet port, said vent port and said delivery port when said pressure at said outlet port acting over said second pressure responsive area equals the pressure exerted by said biasing means and said pressure developed at said first inlet port is typically zero to define a lapped position of said valve and (ii) sealing said first valve seat while opening said second valve seat to provide fluid communication between said delivery port and said vent port at a rate equal to the ratio of said first area and divided by said second area as pressure is supplied to said second inlet port to define a dump position of said valve;
• said valve moving from said dump position to said lapped position when the pressure at said delivery port acting over the second area is equal to the pressure at said first port times the ratio of the second area divided by the first area.