Bleeding brakes on ABS equipped chev PU

slab · BRANT COUNTY, ONT

Bleeding brakes on ABS equipped chev PU

Fordy Acres Car Farm · DORCHESTER, ONT

My uncle was just asking me about this very same issue on his Chevy Avalanche. He said he had it in the shop to get the brakes done and ever since, he's had a soft pedal - he said it still seems to stop OK, but he could put it to the floor if he wanted. The only answer I had was to take it back to the shop that did the work. He said he has taken it back a couple of times and there isn't anything they can do -much like you guys, they re-bled the calipers and sent him on his way....twice.

I'd be interested in the final prognosis/cure so I can pass it along to him - he's getting frustrated and the repair shop is getting tired of him bringing it back.

slab · BRANT COUNTY, ONT

Here's a bit of what I was reading....

Spongy brakes, no matter how much is bled

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I'm having a real problem with the brakes on my 1989 silverado. First a bit of background:

I replaced the master cylinder. Before I replaced it, it would stop the truck, and while holding the pedal, the pedal would slowly drop down. No leaks were found, and after plugging the ports on the master cylinder, I found that the cylinder was the problem. Off to the parts store, I got another one, and bled the brakes every day for a week! But to no avail, the pedal was still spongy. Tried my port-blocking trick on the master again, and I had purchased a bad one. So again to the parts store. Another cylinder is exchanged, and bench bled again, and tested before hooking up the lines - and it was good.

But once again, I bleed and bleed and bleed, and nothing makes the slightest difference. I get clear, clean fluid out of all four bleeders, and clear clean fluid from the bleeder near the master cylinder. (I think it's part of the ABS.) I'm tempted to take it to a shop, but I worry it will be $100 and them telling me either a lie along the lines that a spongy pedal is normal, or that they can't fix it.

My question is this - what obvious thing am I missing? There are no leaks in the system. The calipers and wheel cylinders are new, and they aren't leaking. The truck stopped fine before I changed the cylinder, with the exception of having the pedal go down while holding it at a stop light.

thanks for the help:- )

-Mark

Re: Spongy brakes, no matter how much is bled

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A bad isolation/dump valve in the RWAL system can cause this problem.

Here is a snip from a web site that covers this issue. Use the link below to read about this problem.

Pedal problems
One of the most unnerving failures that can occur with this system is the loss of pedal when braking. The problem may feel like a bad master cylinder, but it may not be the master cylinder. The real problem may be a bad Electro Hydraulic (EH) valve in the rear wheel antilock brake system.

If a small piece of dirt or rust gets into the unit, it may prevent the dump valve inside the EH valve from closing. The dump valve will then leak fluid into the accumulator when the brakes are applied. The misrouted fluid allows the pedal to drop, and the pedal may go all the way to the floor without applying the brakes. No ABS warning light or fault code will be found either because the limited diagnostics on this system can't tell if the dump valve is fully closed or not.
www.aa1car.com/library/abs_kelseyhayes_rwal.htm

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That's it. Very common on these trucks. A lot of us have done the same as you and replaced the master cylinder for sure it was the problem and it wasn't .

Originally Posted by benoit454 View Post
where can I find an EH valve? is it called something else like a dump valve or something?

It is commonly referred to as the isolation/dump valve.

The best web sites with information on this system are no longer available.

There was a site with detailed information on testing to make sure the accumulator in the isolation/dump was filling with fluid causing the sinking pedal. It had great cutaway illustrations.

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I did some research after posting the message yesterday, and it would appear you guys are on the money. I have found used isolation/dump valves for around $200 (!) and have to ask the question: What happens if I run the rear brake line around that valve, take it out of the system?

It appears that the heart of the "valve" is an accumulator, with which I am familiar, a spring loaded piston that absorbs pressure. (Making the brake pedal springy in this case) Shouldn't taking it out of the system leave me with good brakes, with the exception of my RWAL which I am confident isn't working anyway? I can't get a used part around here for less than $200, and of course the used one will likely be just as bad, and I worry what will a new one cost?

Re: Spongy brakes, no matter how much is bled

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Fixed it. The accumulator under the hood was the problem, so I took the hex off the end of that accumulator. When stepping on the brakes, the piston inside would push out. There was a spring about 2 inches long inside the accumulator, and that spring stuck out 1/2 an inch when the piston was pushed all the way in. With a little trimming, I ended up with a piece of tubing 1 5/16" long, the same diameter as the spring. So now the truck has a firm pedal, it stops with no problems. If this fix doesn't work, it looks to me like the best thing to do is take the accumulator out, and just run the brake line from the proportioning valve directly to the rear brakes.

slab · BRANT COUNTY, ONT

This is a common response to...

More than likely you have a brake hose that is nearing the failure point (beginning to bulge when you press the pedal) or you have some trapped air in the system.

Stainless Brake hoses will definitely firm the pedal up, unless you have trapped air and you don't get it out after changing the hoses.

This is all providing you are not loosing brake fluid..... Which is another problem all together.

This the one I think makes the most sense to start with...Air in the ABS

Before you start throwing money at it, do yourself a favor and take the few simple steps I suggested. If you woudl like to do some more reading on the topic pay a visit to dieselplace.com and do a search for Heymccall and brakes. He maintains a fleet for a construction company and has much to say on the topic with his primary recommendations being those I made above. Also be careful if you decide to go the route of brake lines or calipers. IMO you don't really want to open these brake systems up and get air in them. If you get air in the ABS system you have to take it to somebody with a Tech II to bleed it.

poncho62 · ADMINISTRATOR

Good stuff Rich...question though.

That link you gave is for rear wheel ABS....Does the same apply with the 4 wheel ABS that I have on my 97 and you have on your 99? Does the ABS unit look the same?

My 97s brakes were spongy when I got it...Replaced the rear wheel cylinders, front calipers and pads....Much better now, but still doesnt feel quite right.

slab · BRANT COUNTY, ONT

I am not sure of anything at this point poncho. All I've been doing is reading and trying to get info so when I get back to work on the truck I can hopefully fix the problem. My main problem is I just fixed the front end ...completely. 4 ball joints, 4 tie rods, pitman and idler. The truck is extremely misaligned and I can't test drive it. It still needs a safety check as well. My trip permit just ran out. So I'm hoping my mechanic will do the alignment tomorrow then I can do some of the suggestions which is take it to a gravel road and make the ABS come on releasing the air, then re-bleed the brakes. I hate to say it but I don't entirely trust mechanics. After reading the forums, the most common thing is "my mechanic says this is just the way it is". Some will try to fix it the same way as you just got done trying....bleeding the brakes. Even though you might ask can you do the ABS bleed with your scanner....you will get a yeah no problem...then get the truck back in the same condition but your wallet is emptier. My mechanic is pretty good but I've seen them fight with brake bleeding and I didn't see them get out the scanner/tech II gaget. So I will ask him tomorrow but I don't have my hopes up. Again that's why I'd like to try the gravel road thing.

I also had to replace the rear axle seals. I think I would be best off to go back and totally go through the rear brakes. Maybe the cylinders are leaking and I didn't catch it due to the rear seals having made a mess first.

Here's another thread....This is a popular response along with the rear drums not self adjusting properly.

lastly... i have also had issues... with the caliper slides... where people have improperly greased them... using silicone dielectric grease is a NO NO... silicone dielectric grease has dissolved SAND in it.. when heated by the brake system.. the sand comes out of its liquid form.. becomes sand again... and prevents the caliper slides from moving freely... i have found some that would not budge with the 30 ton shop press.. yep i tried..

why.. when the brake pedal is depressed.. the fluid pushes out on the caliper pistons.. this pushes the inner pad against the inside of the rotor.. instead of flexing the rotor.. the caliper slides and the outer pad comes in to equal the pressure exerted by the inner pad.. if the caliper sides are not free.. you will bend the rotor to meet the outer pad.. ever tried to bend cast iron.. it will slightly... you can also find that the caliper stays put. and the outer pad wears as the calipers sides are too tight to allow it to release..

Read more: forums.carcraft.com/70/9526726/automotive-brakes/1999-gmc-suburban-soft-brake-pedal/

-- Edited by slab on Sunday 26th of October 2014 08:01:51 AM

Hog · WOODSTOCK, ONT

Ive noticed that pedal feel has gotten better when I re-adjust the rears on these trucks.

If they wont adjust by driving backwards and applying the brakes hard(repeat a few times), then adjust them up manually using the star wheel. Pedal feel is usualy improved after this procedure, and brake feel is always improved.

If you brake hard enough the ABS motor will engage as well as the weaight transfers OFF of the front wheels. Winter time is the easiest time to induce the ABS system to function on the ice/snow.

Seeker1056 · DOURO, ONT

ok - so here we go

if nothing else has ben touched and you had brakes before any changes like changing out a brake line then there is no issue with calipers, slides, etc etc et cetc

Under the seat of your trucks on the side of the frame is the antilock and proportioning system
Has a bunch of lines and some electrics

This is where the air is located when you have spongy brakes

You may argue all you may wish about where and why - but 99.99% of the time it is there that the issue lies.

With out a power bleeder, you need two people to bleed the brakes, and about 3 hours and a gallon of fluid unless you capture the fluid

I have seen us go around the truck 35 times to finally get the air out

another thing - if you have hydrostatic brakes - that's a whole other can of worms to get to bleed properly

-- Edited by Seeker1056 on Thursday 30th of October 2014 01:03:54 PM

fatchuk · ST MARYS, ONTARIO

All those Gm trucks are a piece of Chit to bleed and there is no two alike from my experiences..Here are a couple things to try if you do NOT have access to a scan tool to home the motors..the abs unit mounted under the cab..gets full of dirt and crap so when you bleed try giving it a couple raps with a rubber mallet or a block of wood..Are you getting fluid out the rear bleeders..is a drum or Disc on the rear..Adjusting the rear will help for sure if drum..and another thing Ihad to do is take it down the road and STOMP on that pedal real hard a couple times and then bleed again..I have had to that a couple times to get a decent pedal..I have also had issues with deteriated front flex lines ..I think they have a new updated flex line with a bigger fluid delivery passage..And don't feel bad.. I know of a hundred shops that have all had trouble bleeding those including many dealers who were stumped and through thousands of dollars in parts at those crap brakes .. Another thing to try is have somebody pump the **** hard and fast for about 15 to 20 pumps and then hold while you crack the bleeder Any one of these may or may not work to get you a pedal..They were the worst piece of Chit for brakes when you had to open the system..Many auto techs have turned to booze after working trying to bleed those things..

-- Edited by fatchuk on Thursday 30th of October 2014 03:46:00 PM

slab · BRANT COUNTY, ONT

Thanks everyone. Truck passed safety and the alignment is done. The brakes aren't too bad but they need to be better. I will start with the rear drums and go from there. Starting with the cheapest things first.

slab · BRANT COUNTY, ONT

I examined the rear brakes and there was several clicks on the star adjuster to get the brake drums snugged up but as I was cleaning things up I noticed the shoes were damaged from the leaking axle seal. I purchased new shoes and since wheel cylinders where only $15 ea and I will be bleeding the system again any way, I bought them to.

fatchuk · ST MARYS, ONTARIO

shoes were damaged from the leaking axle seal.

You really need to replace the drums too ..if they got wet frpm the shoes they will junk..If safety and your family is important and hauling a trailer you need the best brakes you can..new shoes on oil soaked drums will junk in no time..don't care how much you try and clean them ..I would bet that will help your pedal feel lots..

slim · DUNDAS, ONT

Cast iron is porous and adsorbs the oil only to release it when heated, like when you use your brakes. If its a rare drum you can bake the oil out with a few cycles of a few hours each in the oven at around 400F cooling and cleaning between till there is no sign of residue and for christ sake never spray chlorinated brake cleaner on anything hot .Most times its just easier to replace them.

slab · BRANT COUNTY, ONT

Back to the parts store I guess.......

fatchuk · ST MARYS, ONTARIO

How did it pass a safety with leaking axel seals..?? and oil soaked rear shoes....that's a no no..

slab · BRANT COUNTY, ONT

I did the axle seals before the safety and cleaned everything up pretty good before it went for the safety. It was after this thread that I started looking at everything real close and noticed the shoe problem.

fatchuk · ST MARYS, ONTARIO

I am thinking by the time you replace the rear shoes and drums , you might find that your brake pedal problem resolved..You had not mentioned that before with the information on your pedal problem ...If I had known about the oil soaked shoes and drums I think I would have given you different advice ..like replacing the shoes and drums and a good adjustment..Replacing the shoes and drums will most likely solve that..Although those trucks are well known for bleeding struggles ....

slab · BRANT COUNTY, ONT

Your info was a big help as with others fatchuk. I talked with Oldguy71Acadian last week on the subject and he said I should check the shoes out real close. And due to your advice and others, I started at the cheapest spot and found some issues. So all in all ...I am on the right path. There still may be a "bleeding" problem to resolve though. I hope not but the guy I bought it off did replace a line and I know it was him and his buddies that did the work. We'll see.

Don · PRINCE EDWARD COUNTY

Is it air? Might not be. If it is air in the system pump the brakes up sitting in the vehicle. release and let it sit for 30 seconds. Push the pedal again and if there is air in the system it will go way down almost to the floor before resistance is felt. if it is consistently the same but doesn't do that then air in the system is unlikely. Remove completely from you mind that it is ABS. It has nothing to do with that but that is distracting you for no good reason. Vehicles have a rubber disc in the system somewhere in the linkage path between the pedal rod and the master. It is there to provide pedal feel (falsely)to the driver. Typically in newer vehicles it is too soft for my liking and probably yours as well. IE on my wife Dodge Journey I can easily put the pedal on the rug with my 310 lb frame even though there is nothing wrong with the brakes. All I am really doing s compressing the rubber disc as the brakes are on as much as would be required and then some. Can you skid the wheel enough to activate the ABS shudder mode. If so this is the most likely answer and you will just have to get used to the new truck and its different then you are used to feel. If it has drum brakes on the rear check to see if the shoes were replaced but the emergency brake not readjusted. In this case the one shoe is held of the pin by the apply or transfer bar and will give a goofy pedal feel. When I managed large professional garage even with qualified people I used to have to ride herd and kick butt to be sure this was looked after. With air in the system pedal will be dramatically inconsistent. The other two will be consistant pedal but perhaps not to your liking.
don

-- Edited by Don on Tuesday 4th of November 2014 10:27:02 PM

slab · BRANT COUNTY, ONT

Thanks Don. I am planning on keeping this truck for a long time(I always say that) but am willing to invest in it. I have now purchased new drums and wheel cylinders. I also found that when I took the brakes apart the small spring was very rusted and the end (hook) broke in my hand. So I purchased those new as well(spring kit). I am also replacing the lines that go from the wheel cyl. to the top of the axle. I figure that since I am bleeding brakes anyway might as well do any thing that looks like it may cause a problem later. I was experiencing something similar to what you had said about putting it to the rug if you want to. Once stopped if I continued to push the pedal down it would seem to "bleed" down to the floor. That being said , once everything is back together again and bled, I will take it out and and test it and see if I can get the ABS to come on like you have said. There is lots of good info in this thread and I am just trying to work through things one step at a time until it is fixed or as you said "just the way it is going to be" I don't have a lot of experience with ABS or bleeding new brake systems but I think things will work out.

Thanks All
Rich

Don · PRINCE EDWARD COUNTY

All the best with it. Try to ignore it is ABS because it doesn't affect what you are dealing with and may cause you unnecessary anxiety. I realize that may not be easy but it will help you a lot of you can.

don

51 Styline · NIAGARA FALLS, ONT

Sometimes , when bleeding brakes , if you pump the pedal too hard and too fast you can compress the air in the system ,in to small bubbles and will have a hard time getting a firm pedal. That's something my Auto Mechanic teacher told us when I went for my Tech license. He said ,remember, fluid wont compress , air will, and if too much pressure is put on the system bigger bubbles will become smaller and you will have problems with pedal feel. Just a couple of pumps before you crack the bleeder should suffice. Don't know if this is relevant in your case but its worth a mention.

fatchuk · ST MARYS, ONTARIO

Don said..ABS because it doesn't affect what you are dealing with ..

ABS is absolutely a different puppy to bleed..and it does affect how you bleed..to get a good pedal on these trucks..Some are no problem and some are a beech to bleed if the isolation and dump valves get out of whack they need to be rehomed and sometimes they are a two beer and some take a 24.. biggrin I have seen grown men cry over bleeding these ,although it was mostly the owner paying the bill..

v8deuce · OAKVILLE, ONT

Just a fyi that I learned over the years and that is there is three ways to bleed a system one to power bleed that is the best way if you have one available two is to have a buddy pump the pedal,I find this agitates the fluid, mixing air (bubbles) with the fluid so three I like to "gravity" bleed. This takes a little longer but if your working by yourself it's a way to get it done. The master is usually mounted higher than the rest of the system so remove the farthest bleeder from the master and pour yourself a coffee cause it may take awhile before it starts to flow also leave the m/c lid off to allow the fluid to flow. After the fluid starts to come out put the bleeder screw back in (make sure the bleeder is clear of dirt and you can blow through it) you will see a little flow then air then flow again once your happy snug it up and continue with the rest the same way working your way closer to the m/c. If you are replacing the master always bench bleed it before you install it some come with hoses to put the fluid back in as you push the plunger but I like to block the ports fill the master while mounted in a vise and push on the plunger with a phillips screw driver BUT only a 1/4 inch or so and release,if you watch the fluid you will see a few bubbles rise up after the release of plunger,you will need to do this about a million times but when done the plunger will feel firm and there will be no more bubbles. Hope this helps some of you if your concerned about air in the system. smile

Randy

slab · BRANT COUNTY, ONT

Well the truck is back together with new wheel cylinders, spring kit, shoes, drums, and brake lines on both sides up to the flex hose on the axle. I took what people said and put it together. I adjusted the shoes as close as possible and we bled the lines(oh yeah 1st I gravity bled them). We went back and forth a lot before we had no air. Then we took it out for a test. Way better. Still as someone else had said..it seemed to have a different feel to it than my 95 Suburban. We took it down a gravel road just for the heck of it and did a couple 80 to 0 stops making the ABS come on. Everything seemed fine and the ABS worked as you could see them off and on marks in the gravel. And again just to make sure... we bled the brakes again and found just a bit more. We packed it in and called it a day.

Thanks again for all the info....
Now I think that there may be an intake gasket leak to fix....luckily those motors leak out not in. Not sure yet but it shouldn't take long to find out once I start using it. This truck was sadly neglected I think. But it has found a good home now! LOL