Why does the wheel lock light stay on in my Mitsubishi Triton?

A leak in the vacuum pressure pipes is he cause why the green "wheel lock" light in a Mitsubishi L200 4WD Diesel pickup truck is the cause why it's on continuously.

In a friends case the vacuum pressure supply pipe of the switch for slightly higher stationary revs (prob for AC) is leaking. The small rubber pipe is worn by the power brake vacuum pipe due to engine vibrations. When this pipe is leaking there is no vacuum for switching off the front hubs, hence the green "wheel lock" light on the dashboard. Consequently the front axle rod keeps turning.

See the related links for a picture of the malfunction.