Are these procedures free under warranty, or do they want to charge you for work on a vehicle that is no longer under warranty ? If not under warranty, I'd just add a bottle of Chevron Techron fuel injector cleaner twice a year to your gas tank. ( This isn't a diesel, I assume.) As for cleaning the brake lines , that's a bit odd. The biggest problem GMC has had with brake lines is rusting out from the outside, due to using poor materials. ( Although GM insists they don't rust out, they " wear " out. Geezzz....) Is he talking about cleaning off the outside, or flushing out the inside of the lines ? Cleaning off the outside of the lines is a valid idea , provided you follow up the cleaning with a spray of an anti-rust coating such as LPS-3 . It's messy, but it may save you from a ruptured brake line some day . As for flushing out the inside of the brake lines, it might remove moisture and dirt from the brake system. Flushing also might cause a piece of dirt to get lodged in the ABS unit , which will cost many dollars to fix.
