Infamousdaveman

I have an issue that three shops now cannot seem to figure out. I'm still waiting to hear back from GM/Dealership as they engaged their engineers to see if anything more can be looked out but I figured its worth asking in case someone else has experienced this issue or knows what I might be able to check next. What I've replaced so far - New drums, shoes, rotors, pads, multiple master cylinders, and the ABS system was supposedly tinkered with from the second shop. Also the guide pins and adjusters were re-lubed and set/working. When I receive the car back after it gets a full abs brake bleed, the pedal is perfect, great pressure, height, stopping power, etc. Pedal travel is like 2 inches. At some random time, usually within about a week or two after service, I will be trying to stop off a freeway ramp or down a hill going at speed and the pedal will "push through" the 2" point and go towards the floor (say 3-4" inches est.). I have to hold the pedal down to bring the truck to a stop and it will make a "grueling sound" as is slowly comes to a stop. I will also feel some vibration accompanied with it that happens right before it actually stops. After this happens, the pedal usually returns back to normal but it will start repeatedly doing the above and making the pedal worse every time. One time it got so bad, it did trip the brake light. The shops will look at everything, say they cant find a fault, Ill ask them to re-bleed the system and the pedal is perfect again but it only lasts for about a week or two, then the process starts all over. Since this Escalade has a brake system like the trucks, tahoes and yukons from 95 up, I was hoping maybe someone has an idea on what I can look for? Or what could be causing this? 00 Escalade , 147,000 miles, second owner. Thanks so much in advance!