NHTSA Complaint #11587853 — 2016 GMC SIERRA 1500

On a recent road trip my family and I experienced an issue with the Brake Assist on my 2016 GMC Sierra. When I returned back from our vacation I took the vehicle to a 3rd party mechanic that I trusted to perform diagnostics. He advised that I would need a new vacuum pump and stretch belt to correct the issue. Upon receiving this information I advised him to move forward with the repair. What I didn't know was that Special Coverage N182202780 covers this exact repair from GM at no cost to the customer. I followed up with GM to see if I could recoup any of the cost and I was told no because I didn't get the work done at a GM dealership. I find this to be extremely frustrating as I did not know there was a special notice that would have covered this defect, and if I had I would of taken the vehicle to a GMC dealership. I feel that GM should reimburse me for the defect that was known even if I had the vehicle serviced at my chosen mechanic. Any help would be greatly appreciated