how do self adjusting brakes work

for what rear drum brakes ?

When you back up with your vehicle and hit the brakes hard, there is a star adjuster that will spread apart the brake shoes keeping them adjusted up to the drums. There is a lever that rests across the star adjuster that works off of springs when you hit the brakes and moves the start adjuster if necessary. This keeps a good brake peddle height, so you wouldn't have to be pumping the brakes to stop. This is only in the rear of the vehicle.

ok...didn't know if all trucks worked that way...thanks

All vehicles with drum brakes work that way, and a lot of vehicles have 4-wheel disc brakes, that there is no adjustment to them.