Background:
Changed the pads and rotor on the front right wheel. Inside pad was scrapping metal. Outside pad about 1/8 inch left. Used C-clamp to reset piston.

Went to pump up brakes and the pedal got firm as expected but then quickly faded to the floor.
pump and test etc for 20x. Same thing. Pedal quickly fades to floor < 5 secs.

Took for .5 mile test drive. No improvement. No firm braking what so ever.

The system was not opened. Just put on pads and rotor.

Was leaving town and it’s the wifes car - so I trailered it to the mechanic.

Further Diagnosis:
Fluid was bad / dirty etc. Checked the new pads/rotor - OK. Then flushed system and bled all the lines. pedal still fades to the floor.

Pinched off the calipers - eventually all 4 - pedal still fades to the floor.

Replace Master cylinder - pedal still fades to floor.

So far no answers. Anyone have any real data on what might be the issue?

Those with the right solution will be put on my Christmas Card List

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06 Liberty CRD Limited. Stock. 119K.

ABS braking systems can behave somewhat strangely if you are not used to working with them.
1. Did the mechanic who drained and re filled your brake fluid bleed the system properly? ABS systems can be very tricky.

2. This may be just normal operation.
I know with my CRD, when stopped at a light, if I continuously apply force to the brake pedal it will slowly drop. This is a normal thing that the ABS system does.
Most of the time you don't even notice it. It's a very slow pedal drop.

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U.S. Army Retired

Yeah, +1 on this but 5 seconds is very excessive.

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Check for leaks on vacuum assist.
Not detracting from the advice already offered but it is worth thinking about:
What you are descrinbing seems fairly typically describing MC leak-back and that is backed up by your description of dirty fluid.
Don't s'spose you had a Moisture content reading B4 doing this work?
With fluid as mucky as you described it may well be worn cups/seals in the MC and a bit of corrosion.

I've little faith in those pre-built MC's - had three that were Non-functional from the store. Worked for a week only and then failed even though they were properly bench bled. I read it that you retained the original MC properly etc. hence my question about possibility of corrosion or wear. I've had this happen on other marques but those pre-built MC's can repeatedly fail in the same way.
I solved it by putting a kit in one myself. Still working after 80K miles.
The original (after 15 years and 180 000 m) failed in precisely the way you describe on an ABS car.
I thought about rebuilding or getting a new unit - it had me really stumped to the point I kept bleeding every week and then the brakes would be fine with good pedal feel etc - after a week of stop-start driving the pedal would suddenly drop to the floor with no warning and I'd check it all again, bleed it and it would repeat..go tsick of that really quickly.

When I do each wheel cylinder - especially with ABS systems I make a point of (when you use a method to relocate the piston like a G clamp or brake pedal pusher I don't ever just push it back up into the system to sully it - I open the bleed nipple on the slave cylinder and use clear tube to direct the fluid I'm replacing to a jar. I also always connect to the jar lids with leure fitting that are screwed into the lids so they can't spray every where. Amazon have them for cents to a dollar or two.

This way the dirty fluid id not pushed up into the MC or the ABS system. Fluid is should really go one way in a system and that is down towards the slave cylinders and away from the ABS and MC - not up.

I also use large (100ml) syringes to remove the majority of the dirty brake fluid rom the reservoir using a kind-of vacuuming action to pickup debris from reservoir as part of the change process then top off with new brake fluid B4 the system is otherwise touched.
The object is to get it and keep it as clean as possible.

What happens if you do the following?
You did bench bleed the MC?
Place foot on brake pedal engine off - just resting. Start the engine. If it drops to the floor it is likely the MC is faulty.

I get that it drops to the floor:
does it do this with KOEO (Key Off Engine Off)
does it do this KOER (Key on engine running) - I'm guessing from your description it does do this.

Does it pump up with engine off?
I understand that it pumps up with engine running.

Does it hold firm with engine off and foot on pedal?

Good luck.

Additional Info;
I forgot to include the brakes seem fine "normal" for the Heep (jeep) without the key on. They pump up fine. No fade to the floor.

Update:
The mechanic attempted to clean the ABS unit - thinking there was debris pushed back into the unit when the piston was reset.

Now the computer will not communicate with the ABS unit. So anyone with this experience with the ABS unit?

New one from Jeep is ~$600

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06 Liberty CRD Limited. Stock. 119K.

As the ABS has been interfered with the ABS will at least have to be excercised by someone who can run the software.
That would probably mean a trip to the dealer unless you know a wiz with electronics or have a friend with the relevant Interrogator = DRBIII or Starscan. I think the newest version of Giotto (Autoenginuity) may do it.
Suggest you have a read of the service manual - if this part of the brake system is opened or interfered with the ABS motor needs to be cycled. It is a must now that is the case not a maybe.