I buddy of mine has a 2002 Kenworth T300 which mayhave a similar and very common air brake setup. Since new, it took more pedal effort than it should have. Dealer said all was fine and my buddy is a mechanic and has checked every part of it.
After many years and one accident, he found out that the brake manufacturer was trying a new compound that had a tremndously long life but the pads were hard and had lousy bite.
After 150,000 miles the pads were still at >75% but he replaced them anyway. Braking was much, much better.
Since your rig was built during the same time, I would check out all the pressures and then if nothing else, replace all the pads/shoes - which are cheap.