Here are the clues…

I knew my brake pads were wearing thin in the rear… on a long drive, I came to a point where the whole rear end would shudder and rattle. At first, it was not that severe, and I mis-interpretted what I heard as though it were a grind of the pad metal against the disc.
Then my Brake light came on in the Instrument Panel…
The shudder was getting a bit worse…

I ordered and replaced the pads…
But when I drove it, the Brake light was still on, and I still got the shudder…

It seems fine when cold, but after driving for a while, perhaps 30 minutes, if I come to a hard braking point it begins, and then repeats thereafter.

It turns out my Brake Fluid reservoir was WAY low… filling that up turned off the warning light.

Underneath the Pressure Reducing Valve, there is a good deal of moisture which I first thought was coming from a different connection, but now think may be the Valve dripping down… (see pics , ,

My first inclination is to think the Discs may have become too hot and overheated.
I had to replace the Front Discs in January, and I did not replace the rears at that time.

However, I wonder if the Pressure Reducing Valve is failing and there is more pressure going to the rear than ought to be which might cause it to skip…
OR if the reservoir being so low got some air in the system or something….

Because it works fine until it has been driven a while, and because the front discs were recently replaced, and because I just replaced the pads (although it did this just before replacing the pads), I am thinking the discs/rotors need to be replaced, but before I drop the money on that, I wanted to know if with the symptoms described I could have a different problem.

BTW, I replaced the rear shocks just a couple of months ago, but I would not think that is connected in any way.

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1995 850T Wagon 120k