1990 m 923a front axle question

hello all I'm new to military trucks but have been around mack dump trucks for 37 years owning 5 now driving and fixing my question is how do i engage the front axle when i flip the dashboard switch light comes on dash i hear air leaking and then light goes out what can be the problem how to test front axle for lockup

When you shift into low range the front axle will lock up automatically. I have disabled this feature on my truck, but my dash switch still works.

The shifter has a washer that hits a little lever underneath the truck and is simple to disable if you choose to do so.

As far as the dash lever goes, I can't offer any help there.

I am sure someone will be along to help but the maintenance TMs would be a good place to start.

thanks i want to find out why it leaks air and from where also can you feel when axle is engaged without jacking up whole truck