I've had great results using an ultrasonic cleaner for tough deposits. A good unit like the ones at https://www.kaijo-shibuya.com/industrial-ultrasonic-cleaners/ultrasonic-cleaning-tanks/ can get into small passages that are hard to reach with brushes or solvents. Warm cleaning solution combined with cavitation does a great job breaking down grime without damaging parts. Just make sure to rinse everything thoroughly afterward to remove any loosened debris.