So I have a 1995 Thomas, and I drove 200 miles on my purchase date of the bus, and I found that one of my front brake chambers, the big round thing the air line plugs into, the plunger was completely shot and there was no resistance to it, so it essentially wasn't even strong enough to pull the slack adjustor.
Your system works like this. Air line -> Service Brake Chamber -> Slack Adjustor -> Brake S Cam, -> To Drum springs, to brake pads.
All of those items need to work for your air brakes to work properly. If you have broken springs, your pads don't connect when your slack adjusters pull.
If your slack adjustor fails to pull, your S Cam never turns.
If your service brake chamber fails to work, your slack adjustor never sends pulls the S Cam.
My Slack adjusters were also shot, and the rubber spring that tightens the slack with was shot as well. So I myself replaced the Drum Springs, Service brake chamber, and slack adjusters. The S CAM usually does not go bad but it can, but rare.
I'd start with those 3. Have a professional look them over.