Update on Brake problem

Page 1 of 2  
Calling all those who know more about RWAL systems than I do......................
The patient, 88' C1500, 305 TBI, 147,xxx on the clock. The other day I
noticed brakes were about 50-60% of where they normally are. Felt like the rears were out of whack. Pulled both drums. Both were out of adjustment (gotta love GM "self-adjusters"). Snugged up RR, adjuster on LR was frozen so I took it off, broke the nut loose, cleaned threads, lightly greased and put er' back together. As soon as I fired her up, brake light came on and stayed on, and NO PEDAL. Goes right to the floor, I have about 5% of regular braking power.
Here's what's been done in order:
1) Inspected for leaks bumper to bumper (all dry).
2) Checked for adequate vacuum at the power booster (20 in/hg, right on the money).
3) Unplugged RWAL module. No effect.
4) Replaced master cylinder, front calipers and flex hoses (damn bleeders broke off when I tried to loosen them after replacing MC, farkin' flex hoses were cracked so I replaced them as well). Talk about a 30 minute job turning into a 2 hour ordeal............anywho, still no pedal at this point.
5) Replaced power booster and bled the SHIT out of the system. Used a vacuum pump, and pumped 1 pint of fluid through each line. Still no pedal.
Where I am now:
Brakes pump up fine with motor off and bleed well. With brakes pumped up and pedal held, when the motor is turned over the pedal goes straight to the floor. Brake light stays lit. Brakes about 5% of normal.
I have an ABS hydraulic module (isolation/dump valve) sitting in my garage which I'm considering throwing at it based on the following theory. If dump is stuck open, it's not holding pressure to the rears. The proportioning valve sees low/no pressure to rears and adjusts front pressure accordingly. The result, no brakes.
Any thoughts before I go and get covered in brake fluid again?
Doc
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

the
frozen
and
the
hoses
pedal.
the
dump
accordingly.
get a piece of crap in it and stick "open". It is expensive so worth trying to clean. Being that you have one......I say swap er out. You might have to have someone press the pedal while you bleed to get the prop valve back centered when you are done(if its off). I cant remember if that one will go goofy or not.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

adjustment
and
bleeders
up
garage
proportioning
will
trying
to
go
I just replaced it with a reman from Advance Auto ($135, ouch). No effect. It's been awhile since I had to bleed an RWAL hydraulic module but as memeory served: open iso/dump bleeder, press pedal to floor letting air/fluid escape, tighten bleeder, release pedal, repeat till there are no more bubbles. It took awhile but I get her nice and air free, then re-bled all four corners again. F**kin' truck is about one more expensive part from the demoilition derby...................
Doc

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I
like
greased
on
to
If
effect.
re-bled
from
Doc, Don't know if this applies to your vehicle but some ABS modules have to be cycled several times to get all the air out.
Dave
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

the
frozen
and
the
hoses
pedal.
the
dump
accordingly.
Add to the list:
6) Replaced ABS hydraulic module (aka: isolation/dump valve). Still no pedal and the brake light remains on.
Doc

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
pulled the codes yet?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Bill,
It's not the ABS light that's on but the regular brake light. No way to pull codes AFAIK.
Doc
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

ground a-h, turn key on, and wait till it flashes. Very limited number of codes though. In 90 they changed to some soft codes that would be erased when you tried it and set a code though. my grandpas truck was an 88 and I pulled the codes on it. In another post I saw you wrote that the light went out when you disconnected the prop valve. The abs is disabled when that light is on, maybe if you unhooked it, turn the key on till the light went out, then tried bleeding everything again it would reset the dump/isolation valve to their proper position and mystically fix itself? The prop valve needs equal pressure on both sides to reset it self. and if you dump valve is open you would just be pumping fluid into the accumulator.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

when
pulled
when
on,
tried
Bill,
Grounded A-H, turned key on, nothing. After the jumper wire was removed and key turned back on, I get a code 9. A quick internet search tells me that code 9 can be set when A-H are grounded for more than 20 seconds and there are no pre-existing codes. To sum it up, no codes.
IMO it's one of two things:
1) Prop valve is cooked (OE valve, 147k) 2) Iso/Dump is still open. I just replaced the RWAL hydraulic module this afternoon. I don't need to calibrate it or anything to make sure dump is closed do I? Isn't it's base state closed until the ABS computer tells it to open? I bled it out as per the instructions, but they didn't say anthing about calibration.
One last thing I've noticed, with MC resevoir cap off, when pedal is depressed, I get a geyser of fluid from the front brake side of the resevoir, but nothing from the rear brake side of the resevoir. This is what initially made me suspect the ABS hyrdaulic module was shot and the dump was stuck open, but after replacing the hydraulics the problem persists, still just squirting from the front brake resevoir, and brake light stays on (although now it;s flashing code 9's <g>).
Whaddya think, prop valve shot? R&R?
Doc
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Good, my kh rwal manual says that the software changed to that in august 90, your abs ecm must be newer, Older number on ecm would start with 912349k, newer ecms 919220k. Some codes are soft in the newer one and have to be read with scan tool.

I just read the chapter on mc. If the pedal is depressed and released suddenly, i.e. slide foot off side of pedal, that the brake fluid should only shoot up about 3 inches or less, more than that indicates trapped air. I'll think more about this and if its not solved by tommow i'll come up with more ideas
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

and
that
there
90,
read
only
I'll
more
Bill,
Scott M may be on to something. I must have popped the valve off center when I was adjusting the rears. Here's why I think so.
Brakes were kinda goofy Friday. Felt like the rears weren't doing their thing. The way I adjust GM drums is as follows. I pull the drums off and visually inspect, then tighten up the adjuster a few clicks at a time until the drum slides on and off with some resistance. I put the drum back on and hold it in place with 2 lugs and pump the pedal a few times in between adjustments to make sure the brake assembley is centered and the shoes are nicely seated in the drums. After I'm done, I reverse-brake a few times to let the self-adjusters do their thing.
It was after doing this on both sides that my brakes were shot. No lines were cracked so no air got in there in-between when the brakes were working (albeit slightly diminished) and when they were shot.
So, how the f**k do I recenter the prop valve? If you read Scott's post below, he mentions holding the prop valve piston (which currently moves in and out with pedal action (piston pops out when pedal pressed, retreats when pedal is let up. Does your book on RWAL say anything about this at all?
Thanks a million, it's nice to have a few brains in on this one. I've done brakes on RWAL systems a bajillion times and never had this happen before................................
Doc
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The kh book I have only deals with the electrical portion of this abs system.

Most recenter automatically when the system is working properly. I remember reading about some that could only be reset by removing the switch and manually moving the plunger back to the center positon with a small screwdriver.

Not related to the proportioning part of this valve. Only affects the metering or "hold off" valve for bleeding the system correctly. See earlier post for more info on this subject.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

adjustment
and
bleeders
up
garage
proportioning
out. If it goes out it isn't centered. If so, have someone press the shit out of the pedal while its running and bleed a front and a rear to see if you can get it centered. The light will go out if it does return to normal. If you cant get it that way, try and remove the sensor on the valve and carefully push it back. If removing the wire from the prop valve doesnt turn the light off , check contenuity from ground to the sensor, (with out wire on)should be none. If you have continuity you got two problems(if it had two lights, both would be on) did that make since? GL

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The 3 ways of turning the light on, parking brake (dim), prop valve (dimmer) and abs ecm (dimmest). If the light gets brighter when you step on the parking brake, it isnt that. If it gets brighter when you ground the wire to the prop valve, that isnt it either and than your left with the ecm. Ground the aldl a-h and see what codes come up.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Pull the codes. jump A to H of aldl. Codes should flash after about 20 seconds. Count the short flashes after the long flash. Including the long flash in the count. The first count may short but subsequent counts will be accurate. To clear codes I believe the STOP/HAZARD fuse must be pulled. I have a factory service manual for my 92 but I believe its the same. If you need anything out of it let me know and I will send it to your E-mail Address. Good Luck JRE
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

be
you
JR,
See my reply to William's post a few messages up from this one. In a nutshell, no stored codes. It's either the prop valve is cooked or the ABS dump is stuck open, even after replacing the ABS hydraulic module with a new one.
Doc

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

goes
normal.
turn
two
Scott,
Brake light goes off when wire is unplugged. I figured that this was simply due to the fact that with the wire unplugged and the e-brake off, there isn't anything to make the light go on. You're telling me otherwise eh?
On the proportioning valve, there is a large, flat nut on the front of it with a nippley-lookin' thing in the middle that moves in and out with pedal operation. When this nippley-lookin' thing is depressed with a phillips screwdriver and the pedal is pushed the brake fluid comes squirting out pretty good.
Replacing the prop valve was my next step as it's the only damn thing I haven't replaced yet. Seeing as how my brakes were pretty shitty the day before I started tinkering I'm beginning to think that this guy was on his way out anyways, and when I adjusted the rears it helped it give up the ghost just a little bit quicker.
Doc
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

shit
if
wire
simply
pedal
safety thing, if you loose fluid/pressure from the front or rear it cuts out the offending side(front or rear. I dont remember exactly how to reset the little piston back to center in there, but that is what needs to be done. You should be able to remove to sendor and manually center it if need be. What happens is, when it goes off center it lets the wire ground turning your light on. If you remove the sendor or switch or whatever you want to call it you will see how it works, and you will see the little piston with a low or high spot in it that the tip of the switch rides on. Take a little screwdriver or something and push it back to center. You know its right if it turns the light out. The only other thing that will light the light is the ABS or the park brake. It doesnt have a ABS light on it does it? I think it just uses the red "brake" light on that year, no other light. Just FYI, you probably popped the valve off center when bleeding it. When you get it reset, make sure the rears are adjusted up, pump it a few times slowly, (mostly just for fun, and to get a feel of it)start bleeding from the rears first. If there is alot of air somewhere you risk popping the piston in the prop valve again when you stomp the pedal. It been a long time since I worked on this setup, I might have a book on it(actually took a class at the GM training center on this RWAL, but a very long time ago...:) I'll see if I can find the book and see if it say anything interesting :) GL and Ill let you know if I find it. Maybe theres a way to reset it with the little nipple you discribed. Ill look for the book.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I read this on the internet, it must be true! :) /Couldnt find the book......
It sounds as though at some point in time either the rear brakes failed, or more likely, that when they were bled, the valve wasn't held in position so that it wouldn't transfer all fluid to the fronts. The valve can't differentiate between bleeding and a catastrophic failure.
There's a pin that should stick out the front of the valve and must be held with a clamp, vise grips, whatever, while bleeding to keep this from happening. You may have to open the rear bleeders to allow the pin to be pulled out.

and
doesnt
had
it
day
his
out
a
think
rears
the
the
I
nipple
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

light
see
out
there
eh?
phillips
out
I
the
a
the
done.
be.
to
with
little
if
is
FYI,
it
if
let
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.