UPDATE: 2011 Explorer Brake issue - need help with diagnosis

[Aggiesdm]

Hi all, I have a 2011 Explorer XLT with 130K miles on the clock. I've owned the car since new and my daughter is now driving it. She came home complaining of a sinking brake pedal and this was confirmed on a test drive. I immediately thought either a leak at a caliper or a bad seal in the master cylinder or...maybe ABS? So, here is what I've done so far and I'd love any advice possible:

1. Confirmed that there were no issues at any calipers
2. Used AC Delco replacement master cylinder (considered OEM replacement) - bench bled it and then bled again while on the car
   1. Upon inspection of old unit, I saw no evidence of bad seals or fluid blowback
3. Problem persisted. No change in the slowly sinking brake pedal. I test drove it and if I double pump it, it gets better but quickly loses pressure down to the bottom.
4. I took it to a reputable local shop to have them thoroughly bleed the system and diagnose exactly what is going on.
5. Got a call from them today...system bled on machine..problem still persists. Their first recommendation was to replace the AC Delco with Ford OEM unit. Total of $531. I baulked due to the unlikely scenario that both the OEM and the brand new replacement were both bad. Tech said he was going to have a master tech look at it tomorrow. He said it could be the ABS unit (which was my main concern all along but that they wanted to rule out the Master Cylinder first. Again, I baulked at the idea of throwing parts and money away as a process of elimination.
6. I researched more and found that the ABS control unit is not available from ford and is on a long back order, so I think I'm going to get the car home, buy a used unit or aftermarket and replace it myself. Thoughts?

Here is where I need help:

1. Is there a testing process to determine failure of the ABS valve body? I watched a couple of videos on a process for previous generation Fords, but I haven't seen one for the 5th get.
2. Anything else I should be considering? Any Ford techs out there that can comment?

UPDATE: So, the shop calls me and says that after the "master tech" looked at it, he agrees that it is the ABS pump unit. He said they could fix it for $1400 for parts + Labor and I told him to pound sand. If I was to follow their logic and diagnosis, this adventure would have cost me about $3K. Turns out I can source a warrantied used part for about $150 and do it myself. With the new master cylinder, diagnosis and used ABS unit, I'll be under about $600. If I would have gone with my gut, I could have been under $300.

So, the lesson learned is that if you have that slow sinking brake pedal and the calipers, brake lines and master cylinder checks out, its gotta be the ABS valve unit. Save yourself a ton of $$ and do it yourself. I'll report back when once I get it replaced. Hopefully it works out!!

UPDATE#2: Well, I picked up the Explorer. They were cool about its and admitted that they'd been no help, so they actually waived the diagnostic fee and only charged me for the brake flush and bleed. $139 and they offered me a job . I politely refused.

I read in a couple of non-explorer forums that other folks experiencing similar symptoms would get the vehicle up to speed and just hammer the brakes a few times to get the ABS to engage and hopefully free up any stuck valves. Well....I tried it and after about 7-8 times jumping on the pedal as hard as I could, it started to work and the pedal stiffened up and is working like new again. Crazy, but I think the fact that my wife and daughter have driven that car for 10 years and probably never had to engage the ABS, resulted in it seizing up. I think forcing it into action solved the problem! I'll report back if anything changes. All in all, I spent about $250, so I consider myself lucky!