Read every service manual you can get your hands on. The old Cat manuals for dozers, wheel loaders and graders. For excavators get your hands on Hitachi and Komatsu manuals. Other manuals might be old International Harvester wheel loaders. Those all have sections in the books that tell you how the systems in the machines are supposed to work. Deere also used to produce some good training materials like the Fundamentals of Service series training books.