Rear air brakes added in step 5.

Front wheels built and added (sans air brake parts) in step 6 (bottom right).

There should be front air brakes, as shown here.

Trumpeter seems to have an issue with front air brakes. Their initial issue of the M983A2 HEMTT Tractor was also missing the front air brakes. In their later M983A2 LET w/M870A1 trailer kit, they added an additional sprue with the front air brake assemblies.