Correct - here's some shots of it in the car....
Now here's the interesting part (and a confirmation that the ABS Pressure Modulator actually fixed the issue).
I bled the thing like you wouldn't believe. In fact, it was more of a flush then anything else as I ended up putting 2 liters of brake fluid through it. Of course, the master had run dry during the modulator replacement so everything was full of air.
I pressure bled it at first to get it started and then gravity bled the daylights out of it. I then test drove it and bled the master some more until brake operation and the pedal worked perfectly. Now here's the kicker....
I was basing the pedal feel on how the pedal felt prior to repair, which was high and hard. I really liked the feel of the pedal prior to repair. However, after some driving and re-bleeding I was certain I had gotten as much air out of the system as possible, yet the pedal was lower and softer.....
I went down to visit my cousin at the shop and asked him for some of the hoses/fittings used for bench bleeding the master (I was going to re-bleed it that way) but I really felt I might be chasing a ghost as the brakes do work perfectly - it's just that the pedal is not as high or firm as it used to be.
My cousin listened to me describe the bleed procedure I had done and said, literally, that I was chasing a ghost. He said the reason the pedal felt softer was because the brakes were no longer being held on and that it was a confirmation I'd cased the issue. I think he's 100% correct. If there's any air left it's going to be a very small amount...
With respect to trace amounts of air being left, he said drive it for a week, let the air migrate through the system and then crack all the bleeders at that point - if I wanted to be 100% certain there is no air left.
Originally I thought that I would not be able to confirm this issue as 'case closed' due to the intermittent nature of the problem but I think my cousin hit the nail right on the head when he said the pedal should be softer. He likened it to a car with sticky calipers where the pedal is always high and hard and when the calipers are replaced it tends to be lower/softer because the calipers are now free to move.
My conclusion: Case closed. Realistically, we had taken everything out of the equation except for the possibility of a software error on the part of GM (had the pressure modulator not addressed it). Given that the pedal feel and height has changed considerably leads me to believe it's a tactile/tangible sign that the issue has indeed been addressed. In a weeks time I will crack all the bleeders but I already know what I'm going to find - nothing more then the peace of mind that comes from knowing that all the air has indeed been removed from the system
So there you have it. Just to recap this thread in one post;
The original issue was that both front brakes were intermittently being applied at the same time during normal driving conditions
The vehicle had no codes whatsoever
The caliper pistons were checked to ensure they weren't seized/seizing - they were free
The caliper slides were checked to ensure they were free - they were
Given that this occurred intermittently to BOTH front brakes at the same time brake hose failures were immediately ruled out
The master cylinder (and the booster and adjustment rod) were ruled out because I was able to get the vehicle to exhibit the problem in the car port and removing the lines to the master cylinder did not free up either wheel (therefore, the pressure must be applied from lower in the system)
Lastly, given that we know that the pressure is not being held to the front brakes from the brake hoses down, and, given that the master cylinder and everything above it is not holding the brakes on, the only component left is the brake pressure modulator.
And finally, concerns were raised that the brakes could be held on by one of the on-board management systems however this was taken off the table by two facts - one, none of those systems were engaged when the vehicle exhibited the problem, and two, when the vehicle exhibited the problem in the car port the ignition was turned off and yet the front brakes remained on. (those systems were not powered up).
So, if anyone encounters the same issue - that's a quasi-troubleshooting flowchart you can follow. Either way, we now know for a fact that the brake pressure modulator can actually cause these issues to occur.
This is an awesome thread with an awesome write up. Wish you were closure to me as I'd make you my go to mechanic.