Brake Drum Wheel Cylinder problems of the 2009 GMC Yukon Hybrid

In late may 2014 I started getting a brake error message & my brakes would not work properly. I would push down on the brakes & it would be hard to brake. I was barely able to pull over, however I made it to the side of the highway & turned off my vehicle, when I turned it on the brake error was gone & my brakes seemed to work. I took my car to the dealer on may 29, 2014 they said it was the master brake cylinder sensor, but I had to wait because gm had the item on back order. I got the car repaired but the day after I picked it up the same thing happened but this time my brakes failed completely & we had to use the emergency brakes and almost got into an accident due to the failure. I took the car in again & they said they needed to repair the abs brake assembly, so we proceeded to approve the repair & get that fixed. I picked up the car after this 2nd repair for the same issue,but when I was driving off the lot I again got the same brake error message & my brakes again were failing. I did a u turn back into the dealer. My car has been thee ever since. This time around they said I needed a couple more parts one being an adapter gm # 25862232. However, this adapter is not available anywhere in the USA or abroad. I contacted gm & they said they know my cars repair issues but do not have the part and don't know when they will have it.