Do your rear brake cans have two or three air lines going to each of them? Being a 4108, chances are they're DD3s.
If they have three, then you've got DD3s, which release differently than spring brakes.
1. Build the air pressure up to the governor cut-out (about 120 psi).
2. Release the parking brake by pushing the button in.
3. Apply a full service brake application, and hold for 5 seconds.
4. Release service brakes, and see if the brakes are free.
If so, problem solved.
If not, besides what the others mentioned, if you've got DD3s, then the rollers may be "rusted" stuck inside the brake cans. Translation: time to pull the cans and rebuild them, or exchange for rebuilts.
FWIW & HTH, somewhat. . .