C4 w/ Hissing Brakes

'92 Auto
The pedal has recently began to feel hard and i have noticed a hissing sound coming from under the foot well when applying pressure to the
pedal. My guess is that a seal in the booster is leaking, but into the foot well? This a complete booster replacement or a 3 dollar seal?
Anyone else experienced this?
Also, in the past I have had a few brake failures after decenting down VERY steep hills. The brakes worked fine on the hills but after getting onto level pavement, the brakes COMPLETELY failed to work. After a bit of pumping they came back to life and worked PERFECTLY fine for YEARS. WTF? Was unable to trace that problem for obvious reasons but now I DO have a problem.
TIA
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I can't offer any suggestions, but, let me see if I understand this. You had the brakes "COMPLETELY" fail and because they came back after pumping you didn't do anything for "YEARS?" Even with my limited mechanical knowledge that sounds like an accident waiting to happen,.even if you had difficulty tracing the problem. Rob

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Scubabix wrote:

Like I said, the problem could not be isolated. Everything was functioning correctly ie. no mechanical or error code conflicts. 2 different mechanics, $500 later, nada.
I should have fixed the problem by unloading the car as this has been the absolute worst car I have ever owned with regards to reliability.
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Sacre Bleu wrote:

I also need to add that when idling in park, depressing the brake pedal also bumps the idle speed up about 300 RPMs.
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If the idle is going up, then it sounds like stepping on the brake introduces a new air path. In other words, the booster is leaking and letting air in.
Descending down the hills could have simply overheated the brakes, not knowing exactly how steep, what speeds, and how much braking. That could have boiled the fluid, which once cooling on the flatlands would have retracted back into the MC and basically left a gap in the hydraulic link (big air bubble in the lines, calipers, or even MC). Pumping them got the air bubble moved to either someplace that lets the brakes work or maybe even out.
Either way, after boiling the fluid, the temperature at which it will boil again is MUCH lower, and should be flushed out and replaced immediately.
Suggestion: 1. replace booster (or repair if you know how) 2. flush brake fluid completely

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Tom in Missouri wrote:

That's what I thought as well, a slightly ruptured diaphram/rear seal. I only hear the hissing inside the foot well. Don't hear it in the engine compartment.

It was a detour that I had to take a few times, about a 9 mile very steep road. No high speeds, LOTS of hairpins. But I do recall a slight 'pad burn' smell 2/3rds down the hill, which would explain overheated fluid theory. So even after purging the fluids, this situation could potentially happen again! Could it be that I'm running cheap brake pads that generate more heat than better quality pads? I do recall possibly installing 'cheaper' pads the last time i did the brakes. Huuuuummmm....

Thanks for the suggestions! I think you nailed it.
I'm guessing I'll need to flush/bleed the line from the MC to the Modulator first. Then flush/bleed each caliper afterward, doing one axle at a time, right? I've noticed that a pressurizer is commonly used on the MC to bleed the system. Is this better than pedal-pumping? Any other comments on possible snafus is greatly appreciated! I've never done this to a car with ABS.
Many thanks!
Jacques
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I had the same sort of problem with my 93, it was the booster. After you take it out, you will see why you might only hear inside the car and not from the engine compartment. I was lucky enough to find a good used one since everyone from the dealers to the parts stores had them back-ordered. I went ahead and replaced the master cylinder while I was at it. I usually try and do things that I might have to take apart a second time. Also completely drained the fluid and replaced it. What I did was get it close and then took it to a shop to have them bleed it the rest of the way.

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I think if you are running lots of hilly/mountainous country, you can run some DOT 4 fluid which is a bit higher temperature than the DOT 3. However, you can find the racing DOT 3 fluids like Wilwood and Motul to get good high temp performance.
Mercedes also uses a factory higher temperature than standard DOT 3. And some time back, many ran FORD fluid that was a DOT 4 or so rating.
I don't think the DOT 4 messes with the ABS, but DOT 5 definitely does. DO NOT run the DOT 5 silicone brake fluid.
The rotors and pads on the C4s had limitations. The rotors were thinner to be lighter, but they overheat and warp much easier than other years. You should check on the Vette Net email list www.vettenet.org or Corvette Forum www.corvetteforum.com for those who have been autocrossing and doing track events with C4s for recommendations and warnings.
I've never done an ABS Corvette either, so I can't tell you.
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Tom in Missouri wrote:

Thanks for the response/links!
I'll look into the DOT 4 solution as I may need to decent this detour in the future.
Cheers,
Jacques
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I agree with Tom in Missouri about potential boiling fluid - especially if the fluid is old. Once the fluid absorbs water (normal with age) it'll boil at even lower temperatures.
As for the hissing, it's a seal in the booster. And if it's anything like my car, it'll get a lot worse fast.
I've searched all the obvious places for a replacement seal but I think we're stuck with replacing the whole thing. UPS delivered an Autozone rebuilt replacement this morning for $100 + $18 core. Autozone will surely take my $18 core, replace the $3 seal, and sell it to someone else for $100.
btw, it's a pretty normal problem. I've had boosters of all makes start leaking like that. Don't get mad at your Corvette.
-rev
Sacre Bleu wrote:

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The Reverend Natural Light wrote:

Yeah, replacing the whole thing is the way to go. $140+core out here in Los Angeles. Is yours plastic?
Also, have you pulled your booster yet? I noticed that support strut that crosses in front of the booster. Did you have to remove it? Looks like a pain! Also, is there enough slack in the lines going to the MC to allow it to be moved out of the way? Not sure if I want to do the MC at the same time.
Thanks for you input!
Jacques
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Yes to removing the bracket, but the hard part is under the dash.

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Charlie wrote:

Oh, I am intimate with that side under the dash. I just had to replace the turn indicator relay! I'm sure those 2 booster nuts are buried up there.
Cheers,
Jacques
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I just finished the job. All things considered, it was quite easy. It would have really been fun if the girlfriend hadn't dropped by halfway through, intent on speading her lousy mood.
The nuts under the dash were easy. I used a very long extension (~24"+) with a u-joint adapter and a 13mm shallow socket for both. Under the hood, the master cylinder has plenty of slack to swing out of the way. No need to crack open any brake lines. Swing the ECM out of the way and unbolt the frame brace. I didn't remove the brace, but once loose and moved slightly there was room to pull the booster out. There is one bolt for the brace that is a pain in the ass - you'll know it when you get to it.
With the booster out, I used a dial caliper, straight edge, needle nose vice grips, and an 8mm wrench to set the rod depth on the new booster the same as the old one. The instructions detail how to set it, but I just made sure it was the same as the old one.
The new booster (autozone.com - $100 +$18 core, free shipping) looks exactly like the old one. I noticed after installation that the brake pedal is much lower than it used to be, to the point that I had to adjust the brake light switch. Perhaps the input rod was supposed to be adjusted. I like it this way because the brake pedal was so much higher than the throttle before that there was no hope of a heel-toe shift. I might be able to do it now.
So, it's one of the easier and cheaper fixes I've had to do so far. Take your time. Post back if you run into any snags.
-rev
('91 C4)
Sacre Bleu wrote:

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The Reverend Natural Light wrote:

Great, thanks for the beta! Sounds fairly painless.
Now take the GF out for a 'mood adjuster'.
;)
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