Spongy brakes, no matter how much is bled

[Tinker1980]

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

[ChevyTech]

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.
http://www.aa1car.com/library/abs_kelseyhayes_rwal.htm

[Boog]

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 .

[benoit454]

where can I find an EH valve? is it called something else like a dump valve or something?

[ChevyTech]

Originally Posted by benoit454

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.

[Tinker1980]

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?

-Mark

[umwtnt]

I had this problem months ago on a company truck and took it to the dealer. They passed it off as okay. I will filed this one away in my memory bank for the future. Thanks.

[benoit454]

the link with the cut out or at least an inside pic is posted above and I read about it. Just wondering where I can find one to buy. If they cost that much I'll freaking bypass mine too, run it right to the master cylinder.

[Tinker1980]

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.

-Mark

[benoit454]

good info to know I think I'd take mine out, I'd do the test posted above first on mine to see if it's making it drop bad. If mines does I think I'll take out the dump valve altogether.

I just went before winter and put new calipers, pads, wheel cylinders, and shoes, adjusters and all that on the truck and was going to do a MC, but didn't and mine stops good but the pedal travels kinda far down before it stops. I think my pedal goes down about 2 inches or so before they start to grab.

[ChevyTech]

Great Tinker!

Glad you found the problem.

[Daniellane]

im in the same situation on my 98. i replaced the master cylinder. bled the brakes. and the backs bled fine and both fronts barely trickled out? and i have hardly any brakes but a sorta stiff pedal at times and sometimes its goes to floor. any help? or ideas as to what it may be. my dad said it may be the calipers?

[ChevLoRay]

Rather than scab onto a post that the Original Poster resolved, why not start a new one?

Try doing a search and see what pops up, too.

Why does your dad think it may be the calipers? If you read the whole post, you'd be seeing some ideas that may be relevant to your issues.

[70c-10]

Originally Posted by ChevyTech

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

Are the following also names for the same thing:

ABS Hydraulic Unit
ABS MODULATOR VALVE

[ChevyTech]

Are the following also names for the same thing:

ABS Hydraulic Unit
ABS MODULATOR VALVE

The site I posted a link to calls it a "Modulator Control Valve"
I will stick to what General Motors calls it.

Not all antilock brakes are the same.
This thread is about RWAL
RWAL = Rear Wheel Anti Lock

Here is a photo of the Isolation / dump valve on my 1993 Silverado with RWAL.
It is under the master cylinder.