I have a 89 5.7 iroc w/4whl disc. Had it since sept 10. it has ALWAYS had a spongy pedal. It frustrates the hell out of me that I cant skreech to a halt if I neded to. Like many others, Ive had the brake system power bled, got new rotors, pads, etc..Nothing. My engine has a mild cam in ti. So, I was thinking maybe the vacuum was affected by that. Not so much now that I see this is so common. The car stops ok for normal driving but if I need to stop fast, the pedal will still go to the floor. Not god.
Has ANYONE had any real luck yet fixing this problem? One mechaninc in NC told me they were fine..cause they were disc all around, I wasnt gonna get that lock up from the rear. I understand what he was saying but, If you wanted to..you should be able to flat spot your tires at 100mph..There is NO WAY I could do that now. A car is not a performance car if its brakes SUCKS. I will do just about anything to fix this.
So, anybody have any real luck fixing this issue?
Thanks

 

Have you replaced the master cylinder?

If not, and your system is fully bled, then it is more than likely a bad master cylinder.
If you have replaced it with a rebuilt one, it could still be a bad master or there is air in the master.

Your master cylinder is referred to as a step bore master cylinder. It requires special bleeding before they are installed on the car.

http://www.dormanproducts.com/docume...Bore042005.pdf

All step bore master cylinders have a 100 lbs psi bypass valve in them that regulates the volume of brake fluid to the front low drag calipers. Low drag calipers require extra fluid to function properly and if the bypass valve does not function properly, it will not get the proper fluid to the front calipers and it will seem like you have a spongy pedal. Most rebuilt calipers do not replace the bypass valve when they get rebuilt. It is always best to buy a new, quality, name brand master cylinder when replacing the master cylinder.

Low drag caliper links.
http://www.mpbrakes.com/uploads/documents/lowdrag.pdf
http://www.remanbrakes.com/pdfs/lowdragcalipers.pdf

 

Sounds like a bad master cyl.
Also If you haven't replaced all 3 of the rubber flex lines with new then that can be part of the prob.
I use stainless braided lines instead of the rubber stuff.

 

thanks. sounds good.
I think Im gonna do that.

 

I've never NOT been able to fix that.

EVERY TIME, without exception, it's been air in the system (unless of course there was a defective part such as the MC). Usually the mystery air has been in the MC. But I don't "have" my system bled. I do it myself (without power tools I might add). That way I can be POSITIVE I got all the air out. In the case of the MC, it MUST be "bench bled" before being put on one of these cars; because of the angle it sits at, air gets up above where the lines hook up, and it will NEVER come out of there. You could bleed it (using power, weakness, gravity, or w/e) 10 hours a day every day from now til Doomsday, and 20 hours on Sundays just for good measure, and that air will STILL never come out. OTOH, "bench bleed" it properly ONCE, case closed.

 

I also vote for changing the master cylinder

 

if you park the car on a good angle that levels the MC and then bleed the system with whatever method you want then see if that helps. Its less work and might just do the trick. Also I agree with the rubber lines getting changed or swapped to braided ones. My buddy did stainless lines and braided lines all around and his pedal was solid when you put your foot down when driving it.

 

Yup... change the MC, YOURSELF, and "bench bleed" it, YOURSELF, before putting it on; and you can then be fairly sure you got it right. As cheeeeeeeeep as MCs are (like $15), hardly worth worrying about, if there's ANY suspicion WHATSOEVER that it might be a problem... not worth risking your own life and the lives of everyone else on the road, over $15. Then if you want to make a further improvement, try the lines, if you can still get them for whichever system you have (they've been discontinued from some mfrs for some variations of these cars so it might not be so easy).

 

Any updates

 

Nope, the properties of air and angles and all such as that, haven't been "update"d. (speaking strictly as a physicist... there's been a couple of "updates" since then, but nothing to that part of the body of fact or knowledge, that I'm aware of, maybe I'm remiss in checking the daily news in my field?)

What do you have in mind as candidates for "change"? Surely we're not revisiting the 2008 presidential election? Are we "hoping" for "change" or some such?

 

I am getting pretty fed up with this myself. I was told that it's always going to be like that because it's just an older car with older designs. I envy newer cars and how you can just touch your foot to the brake pedal and they want to almost lock the wheels.

Is the master cylinder supposed to squirt fluid back into the reservoir for the entire stroke? My old one did that, and a friend said it wasn't normal, so I got a refurbished one, and it does the same thing.

I'll PAY someone if they can get my brakes to act instantaneously. I have new rear brakes, done two years ago. Last summer I did the front brakes. After that I did the recall kit on the rears. Just today I replaced the rubber lines with Goodridge braided PTFE ones. The pedal is EXACTLY the same and still takes A LOT of travel to get the brakes to do anything. I even bench bled the master a few times.

 

The squirting is normal. And although I do agree that the stock brakes on these cars are not sufficient for the weight of the car, they should not feel spongy. Sounds to me like you just need a better bleed. I noticed a big difference in my pedal feel when I replaced the 20+ year old rubber hoses with Earls Stainless braided hoses. I also sucked a couple quarts through the bleeders to make sure all of the fluid was clean, clear and free from water and air. Also, I use a vacuum bleeder to bleed brakes and have better luck than having a helper pump the pedal while I work the bleeder nipples.

 

I don't agree that the squirting is normal. Once the seal on the piston crosses the hole for the reservoir, it should cease to squirt back into the reservoir. I use a vacuum bleeder, too. If it was a problem with air in the system, the pedal would get firmer with pumping. The pedal is firm, it just engages very far down.

 

If the pedal is 'firm' all the way but doesnt engage untill really low, this would be an impossibility.

A leaking master will not pump up, like would happen when air is in the system. The pedal will only be floppy if the booster is shot. Just went through this last week on my 77 jeep. Pedal felt firmish but no brakes untill the floor or thereabouts. And this problem was intermitant, somedays I would have fantastic brakes. It was a bad master cylinder

My bet is a bad master cylinder, or a small leak somewhere which woukd be noticable with the wheels and drums removed.

I have numerous vehicles vintage 1968, 77, 77, 78, 79, 86, 92, 2013, and have rebuilt brakes on all of them except the newest. All the systems have good brake feel immediatly upon pressing the pedal, so I can assure you people its not only a 'new' car thing. Honestly the old cars have better brake feel imho than new ones.

Newmanstown is an hour and a half or so from me In milford nj. If you want to make the trip I could take a look sometime

 

Agree to disagree on the squirting then. Every GM gar I've ever had did that. I've owned at least a dozen of them and I currently own 4 ranging from 1964 to 2003. They all squirt with the res cover off when you push the pedal. And I'm not sure how you can think pumping the pedal is going to compress air in the system to the point that the pedal gets firmer. Air compresses and immediately decompresses when you let off the pedal.

Do you have a rear drum car? If the pedal is not engaging until it's very far down then maybe you'r shoe adjusters are not working.

 

The cylinder should squirt at first, and then once the seal crosses the hole for the reservoir, it should not squirt anymore.

If the master is leaking internally, it can still build some pressure. There are two hydraulic systems inside of the master, and then you have the combination valve below it. It's not just a simple proportioning valve (aftermarket, fronts teed with full pressure, rear pressure restricted through adjustable valve)...

There are no leaks. My money is on a bad master, since someone who has decades of experience also didn't like how the master squirted back into the reservoir for the entire stroke down. I believe it was a reman, so...yeah...time to just get an entire LS1 brake system.

 

This may or may not help you, but it did work for my 86 Iroc with the rear delco calipers - Two years now - https://www.thirdgen.org/forums/brakes/591331-cheap-proportioning-valve-upgrade.html

 

I already changed the spring in my combination valve. All it did was make the rears lock first. Something like this should not be expected to change the pedal feel.

 

Don't know, made mine tighter... Good Luck finding your issue.. It's always something stupid in the end..

 

I neglected to notice your entire description. I guess if it squirts through the entirety of the cylinder travel then there is probably some blow-by on the piston seal.

 

 

The brakes on my IROC were spongy and very weak when I got the car, I changed out the master cylinder and all the hoses, then did a vacuum bleed on the system; the difference was night and day and I have good brakes with good pedal now.