I'm baaaack.

The $35 econo head gasket job  is still holding, BTW!!!!!!!

Working on a '92 2WD Tracker, 200+K miles.  Brake pedal was slowly sinking to the floor at a stoplight.  No fluid loss over the 6 months it took me to get around to fixing it.

I replaced the master cylinder, but I still have the same problem.  The booster is doing its job - when I step on the brakes and start the car, the pedal pushes back up against my foot.

In the photo there are two pieces - "A" and "B" - one looks like a distribution block, the other I dunno.  I'm guessing the two sensors are for fluid level and for pressure differential between the front and rear?

Also, I think I remember seeing a place to download a manual, but I can't find it now.  Help?

If I am correct A will be for the Abs if it has them. If not I am not sure. B will be your proportioning valve! If the pedal is fading and no fluid lose anywhere I would suspect a bad master cylinder!

If I am correct A will be for the Abs if it has them. If not I am not sure. B will be your proportioning valve! If the pedal is fading and no fluid lose anywhere I would suspect a bad master cylinder!

Ack.  Forgot to mention I just replaced it.  I modified the post to say so.

The only thing I have seen to cause brake fade is a leak or bad master! Is the pedal spongy or does it go right down? Check again for leaks! Did you remove the rear wheels and check inside the boots on the rear wheel cylinders? I know people that have overlooked that! Other then that I am at a loss! Unless the new master is bad! I have seen that also!

did u bench bleed your master cylinder? there might be air still in the system

i would try to do the old gravity bleeding trick or if u have a bleeder tool start from the right rear and bleed till u get fresh fluid and then go to the next wheel its RR LR RF LF

looks like that unknow thing has a bleeder screw and if it has abs most likely its something for the abs

The only thing I have seen to cause brake fade is a leak or bad master! Is the pedal spongy or does it go right down? Check again for leaks! Did you remove the rear wheels and check inside the boots on the rear wheel cylinders? I know people that have overlooked that! Other then that I am at a loss! Unless the new master is bad! I have seen that also!

It sure feels like a bad master.  Nope, not spongy - just a sinking pedal - takes about 15 seconds to sink to the floor with light to moderate pressure.  Pedal pumps back up easily.

To answer the rest of the questions posted - yes, I bench bled it.  No, there are no leaks anywhere.  I bled the system with a one-way check valve bleeder that I use rather frequently (I'm a british car mechanic) until clean fluid came out.  Yes, I found the bleed nipple on the whatever-the-heck-it-is and bled it.  No, the car does not have abs, but one guess is this thing could be some sort of pressure regulator to acheive some sort of psuedo-abs.

If I don't get an answer on this one on what it's supposed to do and how to diagnose it, I'll probably just remove it, re-route the lines, and see what happens.  I'm a minimalist (which is why I love this car) so I'm tempted to go that route anyway, but I'd like to understand why this thing is there.

Really does sound like master cylinder. 

I actually got two defective replacement starters in a row once from Auto Zone; really thought I had that part ruled out as the problem when I'd replaced it....until I'd gone through EVERYTHING ELSE.  Again.  I'll never get those two days back. 

Sucks when the new replacement part is already defective. 

im a ford tech. im always getting faulty parts one time i spent a 2 weeks on a turbo diesel i replaced the turbo it was def bad but still wasnt building boost that well so i smoked it and it was the intercooler replaced that still didnt build boost so i started to replace clamps and such then decided what the heck lets fill the intercooler with water and here there were little ass holes in the intercooler

oh and the simplest thing im the same way i love my carb sammi and my individual carbed vdub 

since you replaced the master cylinder, you now have a mystery.
it sounds like the piston seals are letting fluid bleed back into the reservoir.

i would try another master just to eliminate it!

Thanks, all.

Turns out that thing IS a pseudo-attempt at anti-lock for the rear drums.  I found a 14-page diagnostic procedure for the ABS system at the local library, including how to pull codes from the controller,  who'da thunk it?