Freightliner chassis exhaust brake 2003

Hello all, my exhaust brake on my 2003 Discovery has been operating intermittently for the past year. Technicians have been challenged to remedy the problem. The exhaust valve is new 12 months ago and worked well for a while. Indicator lights on the dash show the brake is operational but the tranny does not down shift nor does the valve close on the exhaust. Does anyone know how to test the valve operation while parked?

thanks for all comments

bob