As I discussed in a previous post, we're getting ready to trade my wife's Sedona for a new car soon, and of course something goes wrong (Murphy strikes). Back in March of this year, I did a full brake job on the van including a new master cylinder. The pedal travel had gone long and squishy, and would not bleed out properly. After the master cylinder change, everything was normal again and remained so until yesterday. We drove the van to a store, and the brakes were perfectly normal the whole trip. After finishing our shopping, I stepped on the brakes to shift into reverse and immediately noticed that the pedal travel had gone long and squishy again.
Can a new (well, 9 month-old) master cylinder fail in such a dramatic and sudden manner so soon in its life? I'm about to head out into the garage and attempt to bleed the brakes; we'll see how that goes.
No leaks anywhere? Rear disks or drums?
The only time I've had sudden onset brake failures were rusted through lines or drum brake cylinders failing.
No leaks, the master cylinder is full of fresh, clean fluid. Rear disks. Heading out to the garage right now to attempt to bleed it.
This is the master cylinder I put in back in March. Supposedly OEM quality. 
I mean, signs point to bad master. New doesn't mean Good, it just means New.
Does it have ABS? There is a rare but possible failure mode where one of the valves in the unit (pressure release, I think) fails and can cause a sinking pedal. I know 3rd generation Eclipses were bad for this, but that was the only application where I've heard of it happening.
There is also the possibility that the brake booster has failed in such a way that it is applying vacuum to the backside of the master cylinder and is damaging them. This offer only applicable to systems where the master cylinder isn't the vacuum seal for the front end of the booster: those types have master cylinders "hardened" to handle vacuum on that end.
Well, the bleeding helped a little. Not perfect, but probably good enough to trade it in. I noticed on my test drive that one wheel locked despite the ABS pulsating.
I've had an ABS accumulator failure on a GC Impreza that also seemed like a failed master cylinder, unfortunately the only practical way to test it is to swap parts.
It does sound like a master going away. Did the googles have anything to say about such an issue on this make and model?
