A brake pressure proportioning valve includes an internal bore and a differential area piston slidably mounted within the bore. Under low-pressure conditions, fluid communication is established between the inlet and outlet pressures. When the inlet and outlet pressure increase,BALANCING VALVES the piston moves against the biasing force of a spring to a displaced position to tend to cause the end of the internal bore to seat against a plug to isolate the inlet pressure from the outlet pressure. Any further increase in the inlet pressure increases the outlet pressure only a fractional amount of the inlet pressure increase due to the differential area piston.Reducing Valves A ring-type check valve prevents the piston from being hydraulically locked in its displaced position. A valve comprising a housing having a bore therein, a fluid inlet communicating with said bore, a fluid outlet communicating with said bore,Bellows Seal Valves a piston means slidably mounted in said bore and having an inlet end face and a differential area means for increasing fluid pressure in said fluid outlet in a proportional relation to increases in fluid pressure in said fluid inlet above a predetermined pressure, spring means biasing said piston means to a first predetermined position in said bore, passage means extending from said inlet end face of said piston means axially at least partially through said piston means for establishing fluid communication between said fluid inlet and said fluid outlet, an annular valve member defined by a portion of said piston means surrounding said passage means, a closed valve seat in said bore for engaging said annular valve member to close said passage means and isolate said fluid inlet from said fluid outlet when said predetermined pressure obtains in said fluid inlet and said piston means is moved in said bore against the force of said spring means away from said fluid outlet to a displaced position by fluid pressure acting on said differential area means, said differential area means including a first effective cross-sectional area exposed to the fluid inlet pressure after said valve means is closed by said movement of said piston to said displaced position and a second effective cross-sectional area exposed to fluid outlet pressure after said valve means is closed by said movement of said piston to said displaced position, said first effective area being smaller than said second effective area, said first and second effective areas being arranged such that further increases in fluid pressure in said fluid inlet above a predetermined pressure causes a proportionate but smaller increase in fluid pressure in said fluid outlet when said piston means remains substantially stationary in said displaced position and the fluid flow rate from said fluid inlet to said fluid outlet is substantially zero, and means operable under normal operating conditions for positively preventing movement of said valve seat beyond a predetermined position in a direction toward said outlet and for permitting said piston means to move toward said fluid outlet away from said valve seat to open said passage means whereby the fluid flows through said annular valve member and the fluid pressure drop from said fluid inlet to said fluid outlet occurs across said annular valve member by operation of said differential area means.

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