I was driving home Thursday night, and went to slow down to turn onto my street, and my brake pedal went to the floor. Still slowed me down enough, but scared me. Pulled into a parking lot, and looked underneath for a brake fluid spot. Had this happen before in my 88 Bronco, and it was a rusted brake line.
No brake fluid puddles, and the master cylinder is full. Hmmm.
Get back in, and start it up. Have full pedal pressure again. Got home fine.
Drove to work and back yesterday, and it was fine too.
I'm going to go take the wheels off and look things over this afternoon.
Someone at work mentioned it might be ABS related. My 91 has only rear ABS, I believe...
Could this be a sensor, or a connector that is shorting out somehow?
Anyone have a similar scare? What was the fix?

 

A hydraulic system that is sealed will always, excuse me, should always hold pressure. Any electrical connections will not have any effect on pressure. If you do not see any signs of leakage, it means that you have a master cylinder that is failing. The piston has a series of seals that keeps the two reserviors seperate (saftey thing) and if these start to fail they no longer direct pressure towards the calipers or wheel cylinders. Instead the pressure is directed back and forth within the cylinder itself. For the most part the pressure does not build up (without pumping) and the reserviors just slosh around.
Rust that is permitted to develop in the master cylinder can cause this.
This fluid attracts water in a bad way, and due to the heat changes related to braking systems, condensation is not a friend. This is how lines and such rust from the inside out.
Anyway, me thinks that your master cylinder is acting up.

 

Gets my vote if no leaks.

 

Well, The way he explained it is that the sensor might be shorting, making the ABS recirculate the fluid, and thus cutting the rear brakes.
Your theory sounds plausible too.
It's damn cold out there... and I can't get the garage cleared out enough to get the Bronco all the way in... Brrrr.

 

If a solenoid valve in the RABS valve body failed you would hardly notice any difference in braking except that if holding pedal pressure, such as at stop sign, the pedal would slowly sink. I would also think your ABS light would have come on although not a certainty.
If you still suspect the RABS valve body it is easy to disable the bleed back system.

 

It's you Master cylinder failing,had the same problem

 

Turns out the inner brake pad somehow slid forward out from inside the caliper. It was halfway out. The caliper piston was pushing half on the pad, and half on the rotor. The piston was trashed and it's seal was ruined. Don't know how it only leaked occasionally. The rotor was pretty trashed on the inside too. I didn't even bother measuring it to see if it could be turned down.
Ended up replacing the caliper and rotor on that side, and all the front pads.
Also found on the passenger side that the bearing retainer lock nut was split, and it had allowed about 1/2" play in the wheel... Yikes! It's a wonder this thing actually rolled straight.
Anyway, All is fixed and better now.