Update on Brakes Part 3

Part 2 was getting a bit long so I decided to start a new thread......................
Went at them again this evening. Drank beer, threw empty cans at my truck,
swore alot, etc.
Anyhow, I took the proportioning valve apart via the threaded end caps and visually inspected the pieces parts. Everything appeared to be in good shape. Removed the sensor and tried to move the plunger back to center position.
Put it all back together (that back cap <spring loaded bastard> was a real PITA to get back on) and here's what I have now:
Brake light stays on and no pedal until I pump the shit out of the pedal (motor running) at which point the light turns off, the pedal firms up and I have about 75% of the brakes back. If I let it sit for 10-15 seconds the light comes back on until I pump em' up again at which point it goes off and I get brakes again.
I'm hardly getting any fluid at the rears, lots from the fronts. From what I understand the metering valve will hold off pressure to the fronts until the backs kick in. Because I'm getting no pressure to the rears, the fronts are holding off giving me a real shitty pedal.
So, no pressure to the rears means one of three things:
1) Collapsed rear flex hose. 2) Iso/Dump stuck open (replaced with new unit a few days ago) 3) Proportioning valve f**ked.
Pretty sure it's number 3. Gunna bite the bullet and order my $206 valve. Anyone know how to verify if the dump is stuck open without opening the ABS hydraulics and rigging up a block to the valve? If it was stuck open I think I'd have an RWAL code set eh?
Thoughts?
Doc
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The light comes on even without touching the pedal after about 15 seconds? Somehow pressure is pushing the plunger off center causing the light to come on. That does sound like the accumulator is building up pressure, then pushing the plunger over after the pedal is released.
>Anyone know how to verify if the dump is stuck open without opening the ABS

I looked at my book and there is a spring closing it. All the codes I read in my book are faults with the ecm or the actual wiring circuits themselves, I dont think it can tell if the valves are stuck one way or the other. Jack up the rear and put it in drive, spin the wheels up and stomp on the brakes, They should slow down gradually if the iso/dump are working correctly, and possibly something is stuck under the dump valve and this could clear it out.

It actually couldn't care less what the backs are doing, It holds off till a specific pressure. Somewhere between 75 and 125 psi.

Have someone hold down pedal before it pumps up and crack open a fitting. Start at the mc and work your way to the wheel cylinder bleeder. Find the one where the pressure has dropped off and thats probably downstream of the problem. Im not sure if that would work on this problem, but its worth a shot.
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#4) AIR in the REAR SYSTEM
Before you buy that valve I would "START OVER" and bleed the crap out of the rear. That combination valve is very simple(not much to fail, especialy since you checked it out.) Start at the master and have someone pump it slowly, (nothing in the system open) make sure they get full stroke ALL THE WAY DOWN. Watch for bubbles in the reservoir. If you see bubbles that means there is air in there. If air bubbles come out, fluid is going in to replace it. Pump it slowly (ALL THE WAY DOWN) till it quits air bubbling. Go to the right rear, remove the bleeder and make sure it is not plugged with shit, clean it out and re-install. Next, bleed: open the bleeder ALOT , BEFORE they start to push the pedal(ALL THE WAY DOWN.)Close the bleeder when they hit bottom. This way you dont have any back pressure on the rears until you get some fluid in there. REPEAT until you start to get good fluid flow out of the bleeder. Go to the left rear and do the same. Then bleed them holding the pedal down and then open the bleeder, then close it when they hit bottom. (ALL THE WAY DOWN) Keep in mind the proportioning valve could cut off the rears at any time. The light will tell you. It would probably be a good idea to put some vice grips on that tit on the prop valve while bleeding to hold it out(or in, depending on who you ask)( Im going with -out-.) P.S. Bottom line: THERE IS MOST LIKELY AIR IN THE REAR SYSTEM. Try bleeding anything connected to the rear(ABS VALVE or WHATEVER) if the above didnt work. Did I mention to have your assistant push ALL THE WAY DOWN on the pedal when bleeding? :) What I meant was: have your assistant push ALL THE WAY DOWN on the PEDAL while bleeding. Please read the above very carefully conserning the OPEN THE BLEEDER B E F O R E the pedal is pushed, close when they get to the bottom, until you get good fluid flow. GL Doc.
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.
Doc, have you tried clamping the hoses, just to see what the pedal feels like with no apply parts moving?
Scrib
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Doc,
I'm sorry, but I laughed out loud when I read your post because I can vision myself doing the EXACT same thing. The picture in my mind is priceless - I wish I could explain it!
More importantly, I wish I could help out with your brakes, but after following your last post, you seem to have covered all the bases my brain would have thunk up.
Snowman

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i'ma feel'n ig nored http://www.babcox.com/editorial/bf/bf60133.htm Scrib
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I have folllowed this with great interest, and I have learned a LOT about modern cheby brakes. And I thank you all.
I would like to make a small suggestion, concerning bleeding, I dont know if it is new to any of you, or how helpfull it is. The idea is from motorcycle brake bleeding.
"pull" the fluid out........ using a large syringe, one pulls the fluid, and hopefully, Air, rather than "push" it. This works well on Bike brakes. We tend to get Air trapped in the piston resevoir area, or so it seems. Also, we push the pads back into the resevoir to expel Air trapped there.
Again, thanks for the education. We had an 86 Astro with lots of pedal travel that between myself and 2 shops, could not get out. Now I know that none of us knew exactly what we were doing (wrong).
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Well, I had the similar problem once and this logic may or may not apply. But who knows. My friend had this 1984 Fiero with a hydraulic clutch. He had changed out both clutch cylinders and could not get a stiff pedal to save his life. I got a hair brained idea that I dubbed "back bleeding" I took his sons medicine syringe and added a lenth of clear nylon hose. I then added brake fluid to the syringe and forced it back into the cylinder and up through the reservoir. Had many bubbles. It worked like a champ and had stiff pedal in about ten minutes. I tried the same trick on my 72 VW Beetle brakes with success. This took a little longer and I used a bigger syringe. I also know of a system that Sears used to use. It involves an air pressurized tank filled with brake fluid. It made an airtight seal at the master cylinder and then all the mechanic did was walk around the wheels and loosen the bleeder screws. Never saw that method fail. --disclaimer-- I DO NOT ENDORSE SEARS AS A VIABLE GARAGE TO DO ANYTHING EXCEPT INSTALL BATTERIES OR TIRES...and I am leary about letting them do that, but I do not own a tire machine so someone has to do it. Now I do not know if the newer ABS have valves in them that prevent backward pressure. If they do, than this procedure may not work. BUT if they do not....who knows.

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