Spongy brakes - 96 GMC

[Russell]

Hey guys!

I was heading home from a quadding trip this weekend when I had a very scary failure on my truck -- My brakes barely worked when I had to panic stop for a deer on the road. I was hauling roughly 3000lbs of quads / gear when I had to stop, and hadn't had any significant problems stopping earlier on.

When I hit the pedal, I had very little resistance, and my brake light on the dash came on immediately. Fortunately, I just missed the deer, but needless to say, I was paying way more attention, and going a fair bit below the limit after that near miss.

The next time I tried the brakes, they worked about half as well as normal, and the light still came on. Within 10 stops of that, I had normal braking power, and the light never came on anymore.

That said, this truck's brakes have always been very poor all around just the way it is. You have to press the pedal half way down to the floor before it starts to do anything, and the only brakes that actually work is the fronts. If you pump the pedal three or four times, you get a solid pedal like you'd expect to have normally, and the truck stops very hard, locks the rears, and ABSs the front. The truck will NOT lock up any of the wheels if you just punch the brakes without pumping them first.

I want to totally repair this now, but I need some advice first.

Is this my ABS unit that is causing the problem? It works properly up front when you step on the brakes and the front tires slide. Is it my drums just so far out of adjustment that they simply don't work anymore? If that is the case, why would I loose everything, even up front, then slowly gain it all back?

Is the more likely culprit my master cylinder becuase of the need to pump the brakes?

Or should I simply replace the master and the springs in the rear? I already have brand new calipers & wheel cylinders, the shoes and pads are brand new as well. When I replaced everything, I am certain I bled them out properly, no amount of bleeding causes any more air to come out of the lines. I had the ABS unit bled by GM just incase, and there was no effect there either. my understanding is that this is a fair common problem with these newer trucks...

 

[Russell]

Nobody has any suggestions / prior experience with this?

 

[RedRooster]

I am having problems with this as well. i will relay my experience, i dont have any real definative help for you.

My application is a 2000 old body tahoe limited. front disk rear drum. Very modest 3 inch lift, and 32's.

I've had issues in the past with no power assistance, i would press brake and you could hear the hiss of leaking vacum from booster constantly while brakes depressed.

Anyway i replaced booster and Master cylinder. Started having problem later that year, so replaced the booster again and it worked better but i was still very dissappointed.

During the last and first major tune up, i told my mechanic about my weak spongy breaking issues. And he told me that all it takes is one hard brake, to pop seals or break shafts or something i cant quite remeber in the master cyclider. Any way he replaced master cyclinder. rear shoes, i change my front pads all the time so no need for those at the time time.

Anyway it is better but i still really dont feel confident with the brakes.

Problem I am having right now is the pedal moves progressively down while i sit and stop lights and eventually clicks and simultaneously throws on the brake dash light. I am thinking this in particular is a vacuum leak in the booster AGAIN. but waiting for something more catastrophic. Before i replace the whole system again.

I have heard that apparently you can put boosters and master cyclinders from 1 ton pickups in to increase braking power? I might try that next.

My 85 k5 with 39's 9inch lift front disks and rear drums, stops way harder than my 2000 tahoe, and i dont really get it.

 

[longbedder]

Check your calipers and rear cylinders for leakage, replacing as necessary.

Bleed your brake system.

Replace your master cylinder.


I would go through the above steps in that order if it were mine.

 

[dremu]

I have heard that apparently you can put boosters and master cyclinders from 1 ton pickups in to increase braking power? I might try that next.

Ahhh, be careful with that theory. More likely to make things worse than better. The General designed your braking system to work in very specific ways, and wildly and randomly changing things can make it worse AND harder to diagnose issues. Been there, done that, got the T-shirt covered in gallons of DOT3.

Vacuum boosters are often replaced for no reason. If it's leaking, yes, it's bad. The hissing, though, may be caused by the natural action of the booster -- I don't remember hearing it on my vacuum boosted trucks, but then they're older (70's) and NOISY

To check the booster, stop the engine, with your foot OFF the brake, and then wait fifteen minutes or so. Come back, and without turning the engine on, pump the brake pedal. You should get two or three good, assisted pumps before the pedal gets really hard (i.e. the vacuum has been bled off); this means the booster is fine.

If you get no easy pumps and they're all hard, the booster is leaky. Look at the vacuum line leading to it and the check valve (plastic round thing onm the booster) as they are a lot cheaper and easier to replace than the booster.

If your pedal is soft, it's an issue with the hydraulic system, NOT the brake booster. The booster is supposed to make pedal effort *lighter*, see, and if it fails, pedal effort will INCREASE, not decrease.

OTOH, if the master is leaky or scored or there's air in the system, the pedal will be soft no matter what you do to the booster.

If the brake light comes on *and blinks*, it's the ABS computer mad about something, so you read the blinkies and figure out what to do based on that, just like you do for the Check Engine light and the engine computer.

If the brake light comes on *solid*, the combo valve has detected a major difference in brake pressure from front to rear. This either means you're way low on fluid (and it's prolly swallowed some air) or there's air in the system due to a leak, either in a brake line or a hydraulic component (i.e. caliper or wheel cylinder.)

Intermittent soft pedal is of course harder to chase down, as with any intermittent issue, but a soft pedal is caused by a hydraulic issue, so a close inspection for leaks followed by a really good brake bleed is in order.

-- A

 

[Russell]

Ahhh, be careful with that theory. More likely to make things worse than better. The General designed your braking system to work in very specific ways, and wildly and randomly changing things can make it worse AND harder to diagnose issues. Been there, done that, got the T-shirt covered in gallons of DOT3.

Vacuum boosters are often replaced for no reason. If it's leaking, yes, it's bad. The hissing, though, may be caused by the natural action of the booster -- I don't remember hearing it on my vacuum boosted trucks, but then they're older (70's) and NOISY

To check the booster, stop the engine, with your foot OFF the brake, and then wait fifteen minutes or so. Come back, and without turning the engine on, pump the brake pedal. You should get two or three good, assisted pumps before the pedal gets really hard (i.e. the vacuum has been bled off); this means the booster is fine.

If you get no easy pumps and they're all hard, the booster is leaky. Look at the vacuum line leading to it and the check valve (plastic round thing onm the booster) as they are a lot cheaper and easier to replace than the booster.

If your pedal is soft, it's an issue with the hydraulic system, NOT the brake booster. The booster is supposed to make pedal effort *lighter*, see, and if it fails, pedal effort will INCREASE, not decrease.

OTOH, if the master is leaky or scored or there's air in the system, the pedal will be soft no matter what you do to the booster.

If the brake light comes on *and blinks*, it's the ABS computer mad about something, so you read the blinkies and figure out what to do based on that, just like you do for the Check Engine light and the engine computer.

If the brake light comes on *solid*, the combo valve has detected a major difference in brake pressure from front to rear. This either means you're way low on fluid (and it's prolly swallowed some air) or there's air in the system due to a leak, either in a brake line or a hydraulic component (i.e. caliper or wheel cylinder.)

Intermittent soft pedal is of course harder to chase down, as with any intermittent issue, but a soft pedal is caused by a hydraulic issue, so a close inspection for leaks followed by a really good brake bleed is in order.

-- A

Well, I am going to go through and replace all of the soft lines in my brake system, the master cylinder and also the hardware in my rear drums. I already replaced both calipers and both wheel cylinders when I did a brake job on the truck a few months after buying it. The truck had spongy brakes before I did the swap, and I power bled it back to the same level of sponginess.

My vac booster works just fine, and is not of any concern, I know it has to do with the hydraulics themselves. I already checked the fluid level and looked for leaks but didn't see any, so I think that the integrity of the system hasn't been compromised, more so that the master isn't delivering the pressure or volume required.

The reason for changing the drum hardware is cause I doubt it has ever been done, and I also have my doubts that the drums are self advancing properly. I've found that if I put the e-brake on, I have a little bit of a stiffer brake pedal than otherwise, so I know that the drums are at least part of the problem.

Regardless, I'll be tackling it tomorrow, and hopefully I can get brakes that at least come close to what my old 80 3/4 ton used to be.

 

[dremu]

Well, I am going to go through and replace all of the soft lines in my brake system, the master cylinder and also the hardware in my rear drums. I already replaced both calipers and both wheel cylinders when I did a brake job on the truck a few months after buying it. The truck had spongy brakes before I did the swap, and I power bled it back to the same level of sponginess.

My vac booster works just fine, and is not of any concern, I know it has to do with the hydraulics themselves. I already checked the fluid level and looked for leaks but didn't see any, so I think that the integrity of the system hasn't been compromised, more so that the master isn't delivering the pressure or volume required.

The reason for changing the drum hardware is cause I doubt it has ever been done, and I also have my doubts that the drums are self advancing properly. I've found that if I put the e-brake on, I have a little bit of a stiffer brake pedal than otherwise, so I know that the drums are at least part of the problem.

Regardless, I'll be tackling it tomorrow, and hopefully I can get brakes that at least come close to what my old 80 3/4 ton used to be.

Sorry, wasn't clear ... wasn't actually directly replying to your post, but the guy saying he wanted to put a 1-ton booster/master in his truck to fix a soft pedal.

Sounds like you're on the right track. I should go amend my post about pros and cons to disc brake to include the fiddly hardware for drums and the self adjusters that don't =))

-- A

 

[Russell]

Well, should I just swap out the whole rear diff and not only get 4.10 gears, but also get a factory locker and disc brakes?

I also have a matching front diff I can swap in as well.

 

[chulisohombre]

Well, should I just swap out the whole rear diff and not only get 4.10 gears, but also get a factory locker and disc brakes?

I also have a matching front diff I can swap in as well.

sounds good.may not fix your brake problem.but will up your performance

 

[Russell]

I pulled the rear drums off, and I think I discovered the problem. The shoes were definitely tight up against the drums, tough to get the buggers off, but after I measured the drums I found out that they are majorly worn out. Spec is .285 as best I can tell, and the drums measured .224.

After removing the drums I found that the shoes are already spread far enough apart that there is a pretty significant amount of space between the shoe's brackets and the wheel cylinder's faces.

I imagine putting brand new drums on and compressing the shoes back together to fit will close that space and eliminate a great deal of the travel in my pedal.

 

[Russell]

Well, just got in from spending the entire afternoon on my brakes. I am absolutely astounded that I put up with how spongy / weak they were before. I discovered a couple things that were leading to poor pedal travel / power though:

1) Bad flex hoses up front. Both were swelling noticeably when I hit the brakes. Replaced all three flex hoses with stainless braided lines made by Russell inc.

2) Springs / self adjusters setup incorrectly on both drums. Most springs were stretched. I replaced all of the hardware with brand new stuff, and downloaded the information that told me how to hook it all up properly from AllDataDIY.com

3) Both drums themselves were nearly 80 times more worn out than is considered acceptable, both drums had cracks in them that would expand when you hit the brakes.

4) The drums were tightened with the e-brake half engaged internally, despite the pedal released in the cab. I removed the e-brake cable entirely and manually returned the e-brake levels back to where they should sit without any engagement inside of the cab, then tightened the star wheel to get the shoes up against the drums, then adjusted the e-brake cable to suit. By having the e-brake half engaged, when you hit the brakes it would literally have to move the e-brake cable before the wheel cylinders would touch the shoe webbings.

5) After fixing the above and bleeding the snot out of everything, I took the truck for a test drive and was simply astounded at the braking potential it actually had. I then blew the master cylinder on a hard stop, and lost all front braking power. So, I replaced the master cylinder, re-bled the snot out of everything and wound up with a phenominally stiff / touchy pedal that I honestly didn't think was possible to attain with my truck!

Anyone with an 88+ chevy that has a spongy brake pedal, DON'T just deal with it and consider it to be a normal thing. Spongy brakes are not acceptable, and there is always a cause for it!