Long story...
2001.5 X-Type with DSC, owned for 4 years, daily driven

Had both rear brake lines leaking, so I replaced them, along with the master cylinder, which I thought was leaking. Upon further reflection, I think I simply had spilled a bit while filling, and the replacement has an issue. Since replacing the lines, I can't seem to get all the air out of the system. It took forever following the JTIS method to bleed the lines. Finally I purchased a mityvac, and pulled a ton of fluid through, till no bubbles. A week later, I'll find bubbles again. I'm wondering if air can be trapped in the ABS, or the new master has a bad seal, which is grabbing air. There is no leak anywhere in the system. That's the only place I can think of it could catch air, and should probably put the old master back in.
There's the +1, and now for what followed from the leaking line replacement...

About a week after getting the car back on the road, the ABS light came on and DSC failure on the dash. Never had that issue before. I pulled the codes, finding C1440 and U1900. Following the diagnostics in JTIS, I checked all the connectors/pins and tested the brake pressure sensor circuit, brake light switch, can bus wires, wiggle tests, battery and charging circuit. Everything tests perfectly. I can clear the codes (strangely they cleared themselves on the dash after my initial testing), but after a series of hard brakes, or 10 mile drive, the codes pop up again. I don't see a way to test the DSC module or pressure sensor directly.

Since the problem doesn't occur immediately, I'm at a loss to pinpoint it. Not having the issue before the leak repair, I find it amazing coincidence if it's unrelated. My intuition is to replace the pressure sensor based on diagnosis, but my gut says if air gets in front of it, that could be the cause. Anyone have any experience that could provide further insight?

 

Silly question, probably...did you remove the calipers? Make sure the bleeders are on the TOP...

 

Never hurts to ask. Nope, didn't replace the calipers. Yes, the bleeders aim for the sky.

Oddly, the codes have suddenly self cleared, and she who must be obeyed reports the brakes feel like new. That was yesterday's news. Today, I could be told she's not driving it again till they're fixed.

I've no faith at this point it's cured. That's for sure.

 

And what do you know...I was right, LoL

 

Final follow up, in case anyone else runs into this issue...

The replacement master cylinder was the entire issue. It was pushing air into the system, and it would appear the pressure sensor doesn't react well to this. Unfortunately, there is no low brake pressure code available to be thrown, so it sends the C1440 brake pressure signal fault instead. The U1900 code was a red herring, that had me convinced to follow down the rabbit hole, since I've had rodent issues (Chicago LoL), and a chewed wire was a real possibility. They love my MAF sensor, dagnabit!

Lesson learned! If you have the code, check for air in the fluid first. If it reappears, check for air again, and if it's making air but not leaking, replace the master. Also, if you do end up loading air through the whole system, bleed the heck out of it. Air likes to get trapped, and I highly recommend using a Mityvac, teflon taping the bleeders, and pulling at least 16oz per wheel, and pumping the peddle slowly many times during. I spent the better part of a day, before I felt confident I had it all out. Well...reasonably confident. I feel the desperate urge to cycle 8oz more per wheel, now that it's been used a solid week, just to be sure.