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  #1 (permalink)  
Old 10-22-2007, 11:31 PM
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Spongy Brake problem

Ok...This is a very old and irritating problem. I have a power-vacuum assist on my four wheel disk brakes on my vette. And they are extremely spongey. The peddle goes all the way to the floor and I have very little stopping power. Its to the piont where I can barely drive it, cause I'm scared I'm goint to rear end someone. First off...YES, I blead the lines(a few times). Then I heard that the corvette diaphrams(in the vacuum booster) leak and that could cause it. So I replaced the booster And master cylinder. Blead the brakes...no problems. Then I was getting off the highway on an off ramp and the pressure disappeared! Which of course put me in a unpleasant (but kinda fun) 70mph turn. Anyway; what do I do to fix this problem. Its starting to get ridiculous! There is a proportioning valve under my master cylinder. Could that be letting air in the lines? Or do I just have to bleed the lines again? Someone please help!!!!

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Old 10-23-2007, 05:55 AM
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How old is the rest of the system?..........Does it have SS calipers, or the stock cast ones?........How old are the flex hoses?

Could be a number of reasons.
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Old 10-23-2007, 05:58 AM
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Spongy brakes are usually a sign of air in the system. Try bleeding again. Gravity bleeding seems to work best for me on Corvettes.
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Old 10-23-2007, 06:00 AM
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I've read on this forum that bench bleeding the MC before you bleed the rest of the system is very important. Have you done it?
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Old 10-23-2007, 07:07 AM
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after using a power bleeder thats all ill ever use.
bled the brakes once and that was it.
http://www.apexperformance.net/cartg...st.asp?scat=26
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Old 10-23-2007, 07:26 AM
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If your rear calipers are not adjusted correctly; the master cylinder will not be able to move those pistons far enough in the calipers to effectively apply pressure from the pads to the rotors.

An indication that this is what is the problem is noticing a lower pedal when applying the brakes. Another is having to "Pump" the pedal to get a brake pedal.

Do you apply the emergency brake when you park your vehicle? This is the only action that will keep the rear calipers properly adjusted. It causes a cam inside the caliper to take up any "slack" in the system. This keeps the piston travel to a minimum. As the pads wear, the clearance increases and the piston has to move an additional distance. Eventually, there is not enough travel available to effectively apply the rear brakes.
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Old 10-23-2007, 04:01 PM
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Was the brake fluid low after you lost the pedal suddenly? If so then you must have a fluid leak somewhere. Master cylinders should always be bench bled before they're installed. I never used a power bleeder so I don't know if that will make a difference. Gravity bleeding works well for the rest of the system. If everything else is ok try putting in a master cylinder with a larger bore. It'll displace more fluid when you push on the pedal and that's what some poorly engineered brake systems need to work well.
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Old 10-23-2007, 04:05 PM
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Whoa, guys... we're getting ahead of ourselves with leaks and air. These are classic symptoms of a master cylinder failure. Just because it was replaced doesn't mean its a good one.
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Old 10-23-2007, 04:59 PM
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A bad master cylinder usually lets the pedal just go to the floor....a bad master cylinder dies not cause a spongy pedal. Thats why I keep thinking air. If the new master cyl. was not bench bled before he put it on.....he will have air.
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Old 10-23-2007, 06:43 PM
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I disagree somewhat... a bad master cylinder almost always fails partially on one of the two bores. It lets it past the first cylinder to where you finally compress the second. It seems like it goes to the floor, but its just going past one to the second. For instance, if you lose the front bore on the master cylinder, the pedal feels like it goes to the floor, but its just going really far and then engaging the rear brakes. Rarely do they both fail at the same time.

Mine right now I can tell is leaking past the rings a bit. It is normal and hard one time, spongy the next, and just today it went to the "floor" leaving me with only front brakes. It actually never got to the floor, but when you're slowing down from 75 for a red light, it sure feels like it.

If its ACTUALLY going to the floor with no brakes then I suspect the reservoir got empty and/or there is air in the master. If its consistent and spongy, I suggest air somewhere else in the brake system. A pretty consistent test is when it gets spongy on you, (conditions permitting) slam on them. If you stop poorly with no undue craziness, its probably air in the lines somewhere. If it locks up one axle, its almost always a master cylinder.
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Old 10-23-2007, 10:10 PM
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What year Vett?

I had a 78' that had a warped rear rotor. The movement of the pads with no fluid pressure allowed air in the lines. Also Vett calipers have 2 bleed screws on each caliper. One inside and one outside.
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Old 10-23-2007, 10:43 PM
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My apologies.... I was just reading over my last post and it sounds more argumentative than I thought when I was typing it... Let me rephrase

"HenryHighrise, you are very probably correct, I just didn't get that impression from his description of the problem."

That sounds better than what I originally wrote
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Old 10-24-2007, 01:56 AM
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well from personal experience, i had a rear wheel cylinder go out, and had a scary time stopping once. spongy. but somewhat like what curtis73 explained.
then i also had a master go bad. only way i could stop was pump pump pump. again it didn't go to the floor, but really really spongy.

my bro's camaro he switched from a manual drum/drum setup to a manual disc/drum setup. we bench bled the master, that was fine. once in the car, just hooking up the master gravity started to take over and the fluids started going through the lines. i put the plugs back in because the rest of the brake system wasn't complete. when my bro finished hooking everything up, sealing all the leaks, he finally set up to bleeding. i told him if he cannot spin the wheels when the pedal is depressed, then pressure is holding. he said that even after bleeding 4 times he still didn't have the pressure to not be able to turn the wheel by hand. then he looked at the pushrod. it wasn't fully pushing the piston into the master. after extending it a few turns, no problems with the brakes now... just another perspective hope it helps.
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Old 10-26-2007, 08:02 PM
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Its a 76 vette. It has the stock cast calipers; and as far as the two bleed screws on each caliper...Its only like that on the rear ones. The front ones have only one. The master cylinder was not bench bled before it was installed (i know, i know...I was un-informed). So could that be whats causing it? I bled my calipers again two days ago, and I can already feel the pressure leaving the pedal. If the master cylinder isn't bled before installation, does that ruin it? And I'm pretty sure my rotors are OK; so I don't think they have anything to do with the problem. And to describe the problem one more time... The pedal goes all the way to the floor, and when its at the floor, has very little stopping power. If I bleed them, they are good for about 100 stops, progressively getting worse each one. I had the same problem before I change the stock booster and master cylinder, and its still there. So by all means gentlemen...please continue...your points are being very helpful. So lets continue so I can figure this out.
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Old 10-26-2007, 08:12 PM
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Please bench bleed the master cylinder, then bleed the entire system starting where the lines connect to the master cylinder.
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