Sudden Spongy Brakes on 2004 XC90 T6

[OregonianGuy]

A bit of a weird issue I am trying to nail down on my 04 Volvo XC90 T6. One day everything was fine, then the next day my brakes were squishy all the sudden. I have done no brake work on it, or any maintenance or touched it in any way that would have caused any issues.

I figured the seal on the master cylinder had to be the cause since there are no leaks or loss of fluid that I can see, so I bought a new one, bench bled it, and installed it. Bled the brakes afterwards and the pedal is still soft. Fluid runs clear after replacing it and bleeding, no apparent air in the system after bleeding them 3 times.

The brake booster seems fine, but I have not done any more than the basic pump the brakes with the key off, hold it down and start the car test. Brakes stay firm with the car off after a couple pumps, and then very, very slowly go down just a tiny bit until i start the car. Once the key is on, the pedal sinks.

The only other things I can think of is maybe I did not get all the air out of the master cylinder, so tomorrow I plan to do the "crack the line open while someone presses the brake" technique to see if any air may be trapped, and failing that, maybe see if i can activate the ABS on a gravel road to see if the ABS module has any trapped air. I do not have the ability to do an ABS bleed with a scanner, as it is not supported so my research shows that it can be done like that instead, in a pinch.

There does not look to be any leaks anywhere, so I am sort of at a loss for right now on where to go from here, if the other two thoughts do not pan out.

Would a problem with my actual brakes be something that would just cause this sort of problem overnight, without any sort of prior symptoms? Like I said, everything was perfect, and then the next day not.

I picked up a scanner on Amazon, the iCarsoft Vol V2.0 just to see if there was anything going on with my ABS system.I got 3 faults - "Brake Pressure Sensor 1, Signal Too Low", "Brake Pressure Sensor 1 And 2. Signal Outside Its Permitted Range." and "Brake Pressure Sensor 2, Signal Too Low".

When I replaced the master Cylinder, I kept the same 2 sensors that were on that one, and they were pretty gunked up. I cleaned them up with some rubbing alcohol and they seemed okay, but maybe I should have replaced them? Is it possible for those sensors being faulty to result in a mushy brake pedal?

[chitownV]

The first question would be if you properly bench-bled the master cylinder. It can take some time to bleed the air out of the master cylinder before installation. A cheap Amazon master cylinder bleed kit can help here.

Next would be the pressure sensors you talked about. If you saw they were gunked up, yes, brake pressure sensors can go out. Remember that brake fluid is hygroscopic, absorbing water, and this can cause corrosion in metal parts.

The only thing is, it's your decision on what you want to tackle first.