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A compression relieve type braking apparatus comprising a slave piston, a master piston, a housing defining a first passageway, and a pressure release mechanism. The first passageway interconnects the master piston and the slave piston. The master piston is responsive to the engine cycle in a manner

A compression relieve type braking apparatus comprising a slave piston, a master piston, a housing defining a first passageway, and a pressure release mechanism. The first passageway interconnects the master piston and the slave piston. The master piston is responsive to the engine cycle in a manner to move between a first master piston position and a second master piston position. The pressure release means is responsive to positioning of the master piston in a manner to relieve pressure in the first passageway when the matter piston (in moving between the first and second master piston positions) reaches a predetermined relief location. The relief of the pressure acts to cause the exhaust valve to retract.

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1. An apparatus to cause an engine to operate in a braking mode, where the engine comprises a piston reciprocating in an engine cylinder, where there is an exhaust valve means which normally opens and closes as part of an engine cycle, said apparatus comprising: (a) a slave piston means arranged

1. An apparatus to cause an engine to operate in a braking mode, where the engine comprises a piston reciprocating in an engine cylinder, where there is an exhaust valve means which normally opens and closes as part of an engine cycle, said apparatus comprising: (a) a slave piston means arranged to move between a first slave piston position where said exhaust valve means is permitted to remain retracted, and a second slave piston position where said exhaust valve means is caused to open; (b) a master piston means which is responsive to said engine cycle in a manner to move between a first master piston position and a second master piston position; (c) means defining a first passageway means adapted to receive a fluid, said first passageway means being operatively connected between said slave piston means and said master piston means in a manner that movement of said master piston means between the first and second master piston positions causes a corresponding movement of said slave piston means between said first and second slave piston positions, respectively, (d) a pressure release means which is responsive to positioning of said master piston means in a manner to relieve pressure in said first passageway means when said master piston mean moving between said first and second master piston position, reaches a predetermined relief location, to permit said slave piston means to move from said second slave piston position to said first slave piston position so as to act to cause said exhaust valve means to retract. 2. An apparatus as described in claim 1, wherein said pressure release means comprises: (a) a means defining a second passageway means carried in said master piston means; (b) a means defining a stationary third passageway means to receive fluid at a relatively low pressure; (c) a connecting means to complete a relief flow connection of said second passageway means with said first and third passageway means, wherein said pressure in said first passageway means is relieved by moving said master piston to connect said second passageway means with said connecting means to complete said relief flow connection. 3. An apparatus as described in claim 2, wherein said second passageway means is disconnected from said third passageway means when said master piston means is in said first master piston position and is flow connected to said third passageway means when said master piston is in said second master piston position. 4. An apparatus as described in claim 2, wherein said engine comprises a plurality of said engine cylinders and said apparatus comprises a plurality of subcircuits each of which is adapted to serve a corresponding one of said engine cylinders, each of said subcircuits comprising at least one of said means defining said third passageway means, a flow connection being provided between said third passageway means of at least two of said subcircuits. 5. An apparatus as described in claim 2, further comprising a first check valve means situated in a conduit means between said third passageway means and said first passageway means, wherein said first check valve means permits fluid to flow from said third passageway means to said first passageway means. 6. An apparatus as described in claim 5, wherein said first check valve means comprises a ball valve means to seat in a valve seat means, said valve seat means being stationary relative to said means defining said first passageway means. 7. An apparatus as described in claim 5, wherein said first check valve means is housed in a spool means which is moveable relative to said means defining said first passageway means, said spool means to assume a lowered spool position in which said first passageway means is allowed to have a flow connection to a sump means, and a raised spool position in which said flow connection of said first passageway means to said sump means is blocked. 8. An apparatus as described in claim 2, wherein there is connected in said second passageway means a second check valve means. 9. An apparatus as described in claim 8, wherein said second check valve means normally permits fluid to flow from said first passageway means to said third passageway means, but only when said pressure in said first passageway means exceeds a predetermined pressure. 10. An apparatus as described in claim 1, wherein said pressure release means comprises (a) a sliding surface means; (b) a means defining a second passageway means having means defining a port means, said port defining means and said sliding surface means to be in sliding engagement with one another, wherein said sliding surface means and said port defining means have a flow stopping position in which said surface means closes said port means and a flow allowing position in which a relief flow connection is completed from said first passageway means to said second passageway means to a third passageway means which is at a relatively low pressure, said sliding surface means and said port defining means sliding relative to one another between said flow stopping and flow allowing positions, whereby pressure is relieved in said first passageway means. 11. An apparatus as described in claim 10, wherein said sliding surface means is a wall means of a master cylinder means in which said master piston means is mounted, wherein said second passageway means is carried in said master piston means. 12. An apparatus as described in claim 11, wherein said second passageway means is disconnected from said third passageway means when said master piston means is in said first master piston position and is flow connected to said third passageway means when said master piston is in said second master piston position. 13. An apparatus as described in claim 10, wherein said engine comprises a plurality of said engine cylinders and said apparatus comprises a plurality of subcircuits each of which is adapted to serve a corresponding one of said engine cylinders, each of said subcircuits comprising at least one of said means defining said third passageway means, a flow connection being provided between said third passageway means of at least two of said subcircuits. 14. An apparatus as described in claim 11, further comprising a first check valve means flow situated in a charging conduit means between said third passageway means and said first passageway means, wherein said first check valve means permits fluid to flow from said third passageway means to said first passageway means; wherein said first check valve means comprises a ball valve means to seat in a valve seat means said valve seat means being stationary relative to said means defining said first passageway means. 15. An apparatus as described in claim 10, wherein there is connected in said second passageway means a second check valve means. 16. An apparatus as described in claim 1, wherein said first master piston position is above said relief location. 17. A method for operating an engine in a braking mode, where the engine comprises a piston reciprocating in an engine cylinder, where there is an exhaust valve which normally opens and closes as part of an engine cycle, said method comprising the following steps: (a) providing a master piston means, a slave piston means, and a first passageway means interconnecting said master piston means and said slave piston means; (b) causing said master piston to assume a first master piston position; (c) responsive to said engine cycle, causing said master piston to move from said first master piston position toward a second master piston position, said movement acting to pressurize fluid in said first passageway means which causes said slave piston means to move between a first slave piston position where said exhaust valve means is permitted to remain retracted and a second slave piston position where said exhaust valve means is caused to open; (d) in response to positioning of said master piston means, relieving pressure in said first passageway means when said master piston means, which is moving between said first and second master piston positions, reaches a predetermined relief location, relief of said pressure acting to permit said slave piston means to move from said second slave piston position toward said first slave piston position so as to act to cause said exhaust valve means to retract. 18. The method as recited in claim 17, further comprising (a) providing a plurality of subcircuits each to serve a corresponding engine cylinder and each comprising at least one of said slave piston means, said master piston means, said means defining a first passageway means, and said pressure release means; (b) utilizing said method to cause said exhaust valve means for said plurality of engine cylinders to open and retract; (c) when said pressure in said first passageway means in one of said plurality of subcircuits is relieved so as to expel excess fluid, shunting a portion of said fluid to another of said subcircuits.