System for signaling potential unintended application of park air brake in a vehicle having a lift platform

1. A motor vehicle comprising:

a park brake operable by an operating device for selectively locking and unlocking wheels on which the vehicle travels;

a pressurized air source;

an air circuit for causing the operating device a) to operate the park brake to unlock the wheels when pressurized air from the pressurized air source, that acts on the operating device through an air line leading to the operating device, acts on the operating device with a nominal pressure that is sufficient to assure unlocking of the wheels and b) to operate the park brake to lock the wheels when air pressure acting through the air line on the operating device is less than some minimum pressure for keeping the wheels locked;

a lift platform and associated lift mechanism that are selectively positionable c) to a deployed position that allows the lift platform to be raised and lowered by the lift mechanism when the vehicle is parked and d) to a stowed position that does not allow the lift platform to be raised and lowered by the lift mechanism;

a selectively operable interlock valve connected in the air circuit for allowing pressurized air from the pressurized air source to act through the air line on the operating device whenever the lift platform and associated lift mechanism are indicated by an indicating device to be in the stowed position and for disallowing pressurized air from the pressurized air source to act through the air line on the operating device whenever the lift platform and associated lift mechanism are indicated by the indicating device not to be in the stowed position;

a pressure-sensitive device for sensing air pressure in the air line leading to the operating device;

and an indicator operated by the pressure-sensitive device when air pressure in the air line leading to the operating device has decreased significantly enough from the nominal pressure, but is still greater than the minimum pressure, to indicate the possibility of imminent locking of the wheels.

2. A motor vehicle as set forth in claim 1 wherein the interlock valve comprises a normally closed solenoid-operated valve that is operated open e) when an engine of the vehicle is running and f) a switch that is associated with the lift mechanism for distinguishing between the lift platform and associated lift mechanism being in the stowed position and in the deployed position indicates that the lift platform is in the stowed positions.

3. A motor vehicle as set forth in claim 1 wherein the pressure-sensitive device comprises a pressure-sensitive switch.

4. A motor vehicle as set forth in claim 1 wherein the air circuit comprises: a supply valve; a double check valve; and a push-pull double check valve; and

the supply valve has a pilot port to which the interlock valve supplies pressurized air from the pressurized air source when the lift mechanism is indicated to be in the stowed position, a supply port that is communicated to the pressurized air source by a supply line, and a delivery port communicated to a first port of the double check valve;

the double check valve further comprises second and third ports communicated respectively with respective ports of the push-pull double check valve;

the air line leading to the operating mechanism being in common communication with one of the ports of the double check valve and one of the ports of the push-pull double check valve;

and the push-pull double check valve being operable to selectively allow and disallow pressurized air supplied from the source through the supply valve to be delivered to the operating device through the air line leading to the operating device so long as the interlock valve is delivering pressurized air to the pilot port of the supply valve.