I am working on a 2007 thomas c-2 saf-t-liner. The parking brake will not release. When the parking brake button is pressed the air leaks from inside dash. We have replace the parking brake valve.
Sounds like a problem with an AMU. They are under the bus towards the rear axle. You will probably need to get an air and wiring diagram for your bus specifically to try and diagnose them.
Do the brake lights com on with the pedal and will the dash come alive with key off with brake activation? We have had bad brake light switches bad in the AMU, usually the first section on the drivers side. The interlock needs to see brake signal to release.
Just do a Google search on "Thomas C2 AMU" and you will find images for them. There are probably about 5 of them sandwiched together in a block. You will have to have your VIN to get the correct one, as they all look very similar, but are ported differently.