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| luke189 | '72 non-pwr brakes | ||
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Guru Newb
 
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Brake pedal fades to the floor overnight, pumps up ok. Been like this for 1 year. Been to 3 corvette shops all of which claim to have found one thing or another (no visable leaks). Over the year I have replaced: first both front calipers, secound all brake lines, third both rear calipers, forth 3 master cylinders, proportioning valve. Guess what the pedal still goes to the floor the next day after being bled. Any input? I'm thinking another master cyl.? On occasion the brake warning light will come on upon very hard brakeing. This is a simple non pwr system how can this be, why can't anyone figure it out? Spent over $2000.00 so far and "no joy". Thanks, Bob.
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Posted on: 2011/7/12 17:09
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| tjpreul | Re: '72 non-pwr brakes | ||
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When the brake lines were replaced, were the metal lines also replaced? My first thoughts are the proportioning valve or a smashed/bent/kinked line.
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Posted on: 2011/7/12 17:53
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Vegas wasn't built on winners. '92 auto, MSD, Banski, Taylor wires, SS lines, C5 rims, 3.07 gears, white gauges, seats from a '96. Motor: Forged rotating assy, Probe pistons, CC valvetrain, AFR heads, Procharger P1SC |
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| Durango_Boy | Re: '72 non-pwr brakes | ||
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Bob, who has bled the system after each repair? Bleeding a C3 sucks. It's a long process and it's difficult getting all the air out of the system. An air pocket at the distribution block, where the sensor is, can cause the warning light. This is also why the brakes pump up fine. You're compressing the air so it's tight briefly but then that pressure bleeds off back into the master. As the air trapped in the system expands the pedal gets mushy again. Considering how much has been replaced I would put my money on poor bleeding. New calipers...of which you have 4 of, hold a LOT of air and it's not easy getting it all out. Same for the new master. They tend to hold air bubbles during the bleeding process and that's even if you bench bleed before installation like you're supposed to. Where are you located Bob? |
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Posted on: 2011/7/12 18:08
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| BillH | Re: '72 non-pwr brakes | ||
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Absolutely. That's all it can be with no visible leaks. One new M/C with bad seals - possible, 3 new ones with problems, no. There's air in there. |
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Posted on: 2011/7/12 18:34
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Every man dies but not every man lives. |
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| biggrizzly | Re: '72 non-pwr brakes | ||
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2011 Memorial Day Car Show Winner!
 
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Welcome Bob! I'm glad you made it here unscathed! I hope the guys here will be able to help get your Stingray working the way it should.
There is a great group of guys here that will help out in areas that they have expertise. Bob contacted our club looking for help with the Vette and I sent him here to the brain-trust for Corvettes. Treat him good!! Don |
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Posted on: 2011/7/12 22:31
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Don Haller Corvette Club of America 94Coupe, 383Stroka, PeteK Trans, 3000stall, 3.54rear, Konis and bigger sways. |
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| luke189 | Re: '72 non-pwr brakes | ||
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No the metal lines were not replaced however they do look new. Thanks I will discuss all these helpful hints with the corvette shop. Regards, Bob
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Posted on: 2011/7/13 0:39
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| luke189 | Re: '72 non-pwr brakes | ||
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Funny you should say that about bleeding, I did hear the same thing more or less. I have to believe the shops know how to bleed the brakes. I will pass your comments on to the next shop and see what they will do. This is very frustrating. I'd do it myself except I am not equiped to get under it. Thanks for all the help you all are giving me. The brakes feel great the first day they are bled but the pedal gets progressivly softer as the days go by until the pedal does hit the floor. I guess I need to ask them all what their bleed procedure is. Let you guys know what happens. Regards, Bob.
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Posted on: 2011/7/13 0:46
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| luke189 | Re: '72 non-pwr brakes | ||
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Thanks Don! I feel better aqlready. We might be on to something. I'll post the results as soon as I can get it back to the shop for re-bleed.
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Posted on: 2011/7/13 0:57
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| Durango_Boy | Re: '72 non-pwr brakes | ||
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Bob, clarify something. Is the pedal getting mushy on it's own just sitting, or is it as it's driven. Initially you indicated that you could pump the brakes hard then overnight it would get mushy. Sitting still is totally different than if that happened while driving. If the brakes get mushy after the car is driven then there is another possibility and not a lot of people think to look for this. Rotor runout, or wobble, can cause your problem. A good shop should have a good dial indicator and base...something they will need to check the runout at each rotor. If the rotor is not true and it's wobbling a bit while you're driving the caliper pistons kind of pulsate like an air pump. What can happen is the caliper pistons start sucking air IN and introducing air INTO the system. Now here's where it gets tricky. Normally we would think that this would cause a drip or a noticeable drop in fluid level but in fact it's almost harder to detect than simply the need to bleed the brakes again. The air that is sucked into the system, in the lines, compresses causing the mushy brakes. As you drive, more and more air is sucked in, and it starts making it's way up through the system to the master, where it is vented normally above the fluid in the master. Air is caught in the system in the calipers and master and you have a vicious cycle. The key is determining which, if all or one, of the rotors are out of round and maybe have some wobble or as it's called, runout. The shop can use a dial indicator to determine if you have this problem and they can use shims to correct the problem if it's not a warped rotor. Fix that possible problem, and then when the system is bled it'll stay free and clear of air. |
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Posted on: 2011/7/13 11:14
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| Matatk | Re: '72 non-pwr brakes | ||
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What Matt just said about sucking air in is very interesting. Older corvettes that had lip style seals on the calipers instead of 0-ring style calipers apparently can suffer from this. Here is a nice article on converting lip seal to o-ring:
http://www.cssbinc.com/images/ads/how ... errebuildinstructions.pdf |
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Posted on: 2011/7/13 11:57
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2002 EBM convertible, Magnusson supercharger, cam, headers, etc. 1989 Corvette...RIP |
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| BillH | Re: '72 non-pwr brakes | ||
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The Stig Moderator
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Bob, what Matt described has certianly been docummented buy a bunch od C3 owners (C2's also). The laterial runout (on the face or the rotor) spec is .004 inch max. Shim the calipers parallel to the rotors,Matt? Just wanted to clairfy. |
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Posted on: 2011/7/13 12:40
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Every man dies but not every man lives. |
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| Durango_Boy | Re: '72 non-pwr brakes | ||
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Elite Guru
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Actually bill the rotor is what gets shimmed. The runout is measured with the rotor secured to the spinning hub. The caliper really plays a very small role since the pads kind of 'float' over the spinning rotor. The factory rotors are riveted as you know, so runout issues would be isolated to bearings. When replacement rotors are installed typically the rivets and removed but never replaced. In some cases people tap the holes and use counter sunk tapered head bolts to secure the rotor after runout has been minimized. Without the runout being checked, the rotor is free to wobble and every time the wheel and rotor are removed it changes. Hence the rivets or bolts. |
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Posted on: 2011/7/13 14:06
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| BillH | Re: '72 non-pwr brakes | ||
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The Stig Moderator
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I didn't realize that people were doing that (we do on the racecars).
I'd think that a new rotor would be true. |
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Posted on: 2011/7/13 14:23
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| Trae1976 | Re: '72 non-pwr brakes | ||
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2012 Memorial Day Car Show Winner!
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Yep, I've replaced all four rotors and had to shim all of them to get the runout down to under 0.005". |
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Posted on: 2011/7/13 14:50
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| luke189 | Re: '72 non-pwr brakes | ||
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Guru Newb
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The pedal gets progressivly soft after bleeding while being driven until the pedal just goes to the floor after sitting for a number of hours or overnight. So I guess what your saying about run out could be a factor. I'll pass this along to the shop I'm visiting on Friday which I've sceduled to re bleed and let you know what goes down. Thanks, Bob.
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Posted on: 2011/7/13 18:42
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| crash | Re: '72 non-pwr brakes | ||
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have who ever is bleeding the system do it with a pressure bleeder. see if it makes a differance.
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Posted on: 2011/7/14 10:58
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