86a 86b 88a and 88b are mounted to a manifold block 90 having suitable passageways formed therein with ports 92a 92b 94a and 94b that are connectable to the pressure lines 74a and 74b the check valves 86a and 88a block fluid flow to the swing actuator 31 while the check valves 86b and 88b block fluid flow from the swing actuator 31 in the embodiment illustrated the check valves 86a 86b 88a and 88b are electrically operated having suitable electric solenoid actuators 96a 96b 98a and 98b respectively the solenoid actuators 96a 96b 98a and 98b are each connected to the sensor device 84 while a suitable connector 85 fig7 is provided to obtain electrical power from the vehicle to operate the sensing device 84 and the solenoid actuators 96a 96b 98a and 98b in the embodiment illustrated the sensing device 84 comprises a proximity sensor 100 which senses the frame 18 when a portion of the subframe 20 is pivoted adjacent to the frame 18 as illustrated in fig7 the proximity sensor 100 is mounted to a bracket 102 which in turn is securely fixed to a pivotal portion 104 of the subframe 20 when the subframe 20 is placed in the working position the proximity sensor 100 provides a suitable electric control signal to each solenoid actuator 96a 96b 98a and 98b to open the check.