Update on Brake problem

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

Reply to
"Doc"
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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.

Reply to
Scott M

Doc, see my post ( Ping Doc...part 2) on the first thread.....may take a while to get up....(long)

fer what eets wert, Scrib Abell

utilized for it's intended purpose, the following is a vengfull tool.............

the easiest way to get a job done..................is to get the shop braggart to "show" you how to do it..................and look astonished when he's through...............

Reply to
Scribb Abell

Doc, see if this link works......i fergits i had it

Fwd: Follow a Diagnostic Process When Curing a Low Brake Pedal =A0=A0

snipped-for-privacy@webtv.net Subject: =A0=A0 Follow a Diagnostic Process When Curing a Low Brake Pedal, Bill Williams, Brake & Front End, June 2001

hopefully the link lives on.... Scrib Abell ~fergits what he fergits~

utilized for it's intended purpose, the following is a vengfull tool............. the easiest way to get a job done..................is to get the shop braggart to "show" you how to do it..................and look astonished when he's through...............

Included Page: Follow a Diagnostic Process When Curing a Low Brake Pedal, Bill Williams, Brake & Front End, June 2001

Reply to
Scribb Abell

accordingly.

Add to the list:

6) Replaced ABS hydraulic module (aka: isolation/dump valve). Still no pedal and the brake light remains on.

Doc

>
Reply to
"Doc"

proportioning

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

Reply to
"Doc"

Doc, one more try on the link

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Scrib

Reply to
Scribb Abell

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

Reply to
Dave Brower

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

Reply to
Scott M

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

Reply to
"Doc"

""Doc"" wrote in message news:1wQbb.12415$ snipped-for-privacy@nwrddc03.gnilink.net...

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.

Reply to
ScottM

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.

Reply to
Scott M

Scott,

The brakes went from slightly diminished to zero with no lines broken for bleeding. They were at about 65% of normal when I pulled the drums and tightened them up, and zero after I put the drums back on and fired her up. In the process of adjustment I pump the pedal a few times between adjustments to get the brake assembley centered and get the shoes seated in the drums. This must have popped the prop valve off center.

So what yer' saying is, get the pin (aka nippley-thing) pulled all the way out, hold it in place, then bleed the brakes again right?

The prop valve itself is dealer-only item and $206, so I'm going to try to get this one fixed before I spend more money on this damn thing.

Thanks so much for your input thus far,

Doc

Reply to
"Doc"

Still with ya Doc... Been there and did that too, bought the new one and same thing. This might sound crazy but I'm wondering if the inside of the metal brake lines may be deteriorating in a way that might be allowing air pockets..... Damn, theres nothing original left but that on mine. This problem seems to only be occuring on older trucks. Im still considering taking mine to the dealer and let them try one of those power bleeders on it. Check this out Doc, I think Mac has one too.

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Reply to
CCred68046

Did you try to recenter the prop valve in your endeavors? If so, how is it supposed to be done?

Doc

Reply to
"Doc"

My Goodwrench buddy took that apart twice and checked it, said it was perfect but we bought a new one of those too. I suspect it was fine as was the ABS valve I bought. I just read through the entire service manual brake section and searched the web for you and didnt see a damn thing about it. I really doubt that is the problem because we clamped off ALL the lines and opened them one at a time and tried to bleed it that way. We did this test so many times I had to replace the rubber flex lines, hell at one point i was thinking one of those might be balloning from the pressure and letting the pedal drop. Even if the prop valve is in the wrong position my system was full of fluid and the pressure would HAVE to go somewhere, it does not vent fluid!. Thats how we wound up changing masters 5 times. My only advice right now would be to find a power bleeder, a lot of shops have them. There has got to be air trapped in there and a lot of it, more than the ABS valve and the prop valve put together could hold. I'm sending copies of this to my GM buddy and all he's said so far is "Ohhhhh man".

Reply to
CCred68046

I don't think it was air in the system that caused the initial problem, although I may have some in there now. The brakes went from 65% to 5% after I adjusted the rears with no breaking of lines. There is no way air could have been accululating in the system since I last did the brakes a few years back as I'd have noticed a fluid leak somewhere. So, air may be a problem now, but the big culprit remains a mystery.

Doc

Reply to
"Doc"

YES, but.....I dont know if it will center the piston in there or not by doing this. Remember, your brake light will not be on if it is right.(must be plugged in). Try to remove the switch or take the whole thing apart if need be to center it. Then bleed while keeping the pin out. Do you understand the piston part? do you get LOTS of fluid from all the corners?

Reply to
Scott M

Reply to
Scott M

Doc The combination / proportioning valve 'pin' should move out with pedal force applied and more importantly retract when pedal is released. This indicates front brake is pressurised. To recenter light, block the pin from extending (gm or snap on or Mack probably have such a fixture) then push hard on pedal to re seat switch. Be careful though for you could set the switch to rears failed position. The pin protruding from front side of valve is not the switch trigger: Trigger is buried deep inside the assembly. Proportion/ combiation valve is probably toasted due to its age Respectfully Rolf

Reply to
nozel

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