I have a '93 Chevy Suburban K2500 4x4. When I look up parts for it, I
always need to tell them that it's "over 8300 lbs" or 8600 lbs. I
believe I have 4 wheel ABS on this truck. Recently, I blew out a flex
line in the LF wheel, and by the time I got home, the entire system
was completely drained and the brakes were absolutely gone.

So... I bench bled the master cylinder, replaced the flex lines,
replaced the front calipers, and went to bleed out the system. I have
a Haynes book that generally has some good information in it and it
says I can bleed out the ABS, but it's sketchy in one spot. It says
(paraphrasing here):

* Push in the needle valve in the front of the combination valve and
hold it in for the entire process
* Bleed (in the usual manner: RR,LR,RF,LF)
* Bleed the "Isolation/Dump valve"
* Re-Bleed at each wheel again
* Release the needle valve
* Done.

The problem is that there is no description of where the "Isolation/
Dump" valve is, and it is not shown in any pictures. There is a chunk
of metal on the fender that I call "an ABS unit" which I assumed was
what they were talking about - it indeed has 2 bleed nipples on it. It
also has the lines from the combination valve going to it, and then it
has 4 lines going out (1 to each wheel). So I went to bleed it and no
fluid would come out of the unit at all. Also, pressure on the brakes
did not change at all when opening/closing the bleeder screw on "the
ABS unit". After a number of tries, I gave up on it and just re-bled
the wheels, and took it on a drive. After you start it up, the ABS
light comes on and it stays on. The brakes work okay, but there is
absolutely no ABS going on (tested on some ice).

So does anyone have a clue as to what the "isolation/dump" valve is
and how you bleed it? Is it the thing I'm calling "the ABS unit"? If
so, why couldn't I get fluid out of it?

TIA,
--Jeff