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

A motor vehicle comprising:

• a park brake for wheels on which the vehicle travels;
• a pressurized air source;
• an air circuit for controlling an operating device for the park brake to unlock the wheels when pressurized air from the source is acting on the operating device through an air line leading to the device with nominal pressure that assures unlocking of the wheels and to lock the wheels when air pressure in the air line is less than some minimum pressure for keeping the wheels locked;
• a lift that is selectively operable to an out-of-stow position that allows the lift to be used when the vehicle is parked and to a stowed position that does not allow use of the lift;
• a selectively operable lift interlock valve connected in the air circuit for allowing pressurized air from the source to act on the operating device whenever the lift is indicated to be in stowed position and for blocking the air line leading to the device whenever the lift is indicated to be in out-of-stow 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 from the nominal pressure, but is still greater than the minimum pressure, to indicate the possibility of imminent locking of the wheels.