The neutral override circuit of this invention includes a variable displacement pump controlled by a servo fluidly connected to a displacement control that is biased to a neutral position. A charge pump is connected to the pump and the displacement control. A solenoid-operated neutral override brake control valve is connected to the charge circuit upstream of the displacement control valve and is normally biased to an open position. However, the solenoid urges the valve to a closed position in a response to a signal indicating that the brake has been released. In the open position, the valve dumps charge flow to the pump case until the supply pressure to the displacement control is inadequate to overcome the centering bias of the displacement control. The characteristics of the brake control valve can be set so that some minimum charge pressure and lubricating flow are still provided to the rotating elements of the pump.

A spring-applied, pressure-released parking brake is controlled by an electrohydraulic control system. A solenoid operated parking brake control valve and a manually operated tow control valve control communication between the engine driven pump, the sump, the operator driven pump and the release chamber of the parking brake so that the parking brake can be released and applied during normal operation of the engine driven pump and so that the parking brake can be released by operation of the operator driven pump when the vehicle is shut down. A transport sump valve and an inlet check valve operate to prevent the parking brake from engaging while the vehicle is moving too fast, as a result of operator error, hydraulic failure or electrical failure.

A simplified brake control system comprises a source of pressurized fluid, a spring-set brake with a hydraulic release mechanism and a control valve connected between the brake and the source. The control valve has a valve body and a flow control mechanism in the body and mounted for movement between a first (solenoid-energized) position and a second (solenoid-de-energized) position. A one-way check valve is built into the valve body. How the system operates depends upon two factors, namely, (a) whether the valve flow control mechanism is in its first or its second position, and (b) the relationship of the pressure of the source, i.e., the first pressure, and the second pressure in the release mechanism. When the flow control mechanism is in the first position, the release mechanism may be pressurized for brake release. And when such mechanism is in the second position, the release mechanism is vented to the reservoir through the brake port, the second passage and the drain port. A new method for controlling a brake is also disclosed.

A park mechanism for an automotive transmission having an output shaft comprises a rotatable lock ring structure connected to the output shaft, a non-rotatable lock ring structure positioned in proximity to the rotatable lock ring structure, and a pressure mechanism (preferably, including a spring-biased piston within a pressure chamber) responsive to application of pressure thereto for moving the non-rotatable lock ring structure to a disengaged position for non-engagement with the rotatable lock ring structure, and responsive to the release of pressure therefrom for moving the non-rotatable lock ring structure to an engaged position for engagement with the rotatable lock ring structure. Preferably, the rotatable lock ring structure is provided with at least one receptacle or tooth, and the non-rotatable lock ring structure is provided with at least one tooth receivable in the receptacle or tooth.