c3 brakes

Hello,
I'm having a problem with a 74 I picked up this spring. The rear brakes are practically not working. The front brakes work great. I get a lot of
travel in the pedal and it seems soft. I replaced the rear calipers as one of them was hissing and bubbling brake fluid. I've bled the system both with a vacuum pump and manually. The master cylinder looks very new, but I'm not getting much fluid out of it for the rear brakes when I crack the line open and depress the brake. Could this be a booster problem, I think it's the original? Or is a bad master cylinder more likely? Any insite would be helpful.
G
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G wrote:

I would check the brake lines all the way down the line. I am not really farmilar with the brake system on your year car, but there are junctions of the brake lines that spilt the line into two lines or a balancer somewhere in the front of the car, it is not uncommon for them to get clogged with dirt. check all these connections and then see it the line it free then. Now if the rear end system goes into one side then out and goes to the other. if that is the case check to make sure the preesure going into the main side line is correct because it could be the spilter inside the rear that isn't part of the calipers that send the preesure over the the other wheel.
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Vacuum bleeding of C2 and C3 disc brakes can draw air into the system by sucking it past the piston seals. Avoid using this method.
Manual bleeding I assume is having you bleed the calipers while a buddy pumps the pedal. This can generate air bubbles by pumping too fast or hard. With patience, it works ok.
Gravity bleeding is my preferred method. Fill the master cylinder, crack open a bleeder on a caliper and let it run until no air bubbles are seen. Very slow.
Front calipers have only one bleeder each, on the inside up high.
Rear calipers have two bleeders each, on the upper end, both inside and outside.
All SIX must be bleed.
If the car has not been restored or had the brake system reworked, then check the brake hoses. They can deteriorate and act like a check valve to mess up the braking. Visibly, they look ok. The safe way is if they are old, just replace them.

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I did bleed all 6 bleeders, outer first on the rear. By manually bleeding I do mean having a buddy press and release while I open and close the bleeders (slow even strokes) I tried gravity bleeding, but nothing comes out (yes I waited an hour). I'm not sure if I have a clog, or another problem. When opening the line at the master cylinder and pressing the brake pedal, barely anything comes out, as I said though, the master cylinder is fairly new (and it is a Delco, so are the new calipers). Also, what is the valve/device for about 2 feet from the master cylinder on the rear brake line? Should I look to borrow a pressure bleeder to take care of this?
G

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That's a proportioning valve to, guess what, restrict flow to the rear to avoid over braking the rear. Could be your culprit, clean, rebuild or replace it, good thing to do anyhow. Bleed the lines while you have it off with a jumper to see if you have proper flow.
--
Dad
05 C6 Silver/Red 6spd Z51
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Huh, ok it is a proportioning valve then, only the rear line goes to it, the front line goes to a different valve. I'll give it a try and see what I come up with.
G

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Old rubber flex lines deteriorate inside, swell up, and will block fluid. I had to replace them on my '75 because they were causing the brakes to hang up. A cheap fix. Good luck.

bleeding
(yes
valve/device
to
off
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If you happen to solve your problem please post how you did it. I have a 74 with the same problem.
Nick
did bleed all 6 bleeders, outer first on the rear.&nbsp; By manually bleeding <BR>&gt;&gt;I<BR>&gt;&gt; do mean having a buddy press and release while I open and close the <BR>&gt;&gt; bleeders<BR>&gt;&gt; (slow even strokes) I tried gravity bleeding, but nothing comes out (yes <BR>&gt;&gt; I<BR>&gt;&gt; waited an hour).&nbsp; I'm not sure if I have a clog, or another problem. <BR>&gt;&gt; When<BR>&gt;&gt; opening the line at the master cylinder and pressing the brake pedal, <BR>&gt;&gt; barely<BR>&gt;&gt; anything comes out, as I said though, the master cylinder is fairly new <BR>&gt;&gt; (and<BR>&gt;&gt; it is a Delco, so are the new calipers).&nbsp; Also, what is the valve/device <BR>&gt;&gt; for<BR>&gt;&gt; about 2 feet from the master cylinder on the rear brake line?&nbsp; Should I <BR>&gt;&gt; look<BR>&gt;&gt; to borrow a pressure bleeder to take care of this?<BR>&gt;&gt;<BR>&gt;&gt; G<BR>&gt;&gt;<BR>&gt; That's a proportioning valve to, guess what, restrict flow to the rear to <BR>&gt; avoid over braking the rear. Could be your culprit, clean, rebuild or <BR>&gt; replace it, good thing to do anyhow. Bleed the lines while you have it off <BR>&gt; with a jumper to see if you have proper flow.<BR>&gt;<BR>&gt; -- <BR>&gt; Dad<BR>&gt; 05 C6 Silver/Red 6spd Z51<BR>&gt; 72 Shark Black/Black/4spd<BR>&gt; <BR><BR></BLOCKQUOTE></BODY></HTML>
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brakes
both
but
the
insite
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'Testing the booster is a simple operation of pressing the pedal and seeing if it holds or descends to the floor. It it outlined in all the manuals for the car.'
REPLY: On my 1970, this is happening all of a sudden ; pedal goes down to nearly floor level. No brake boosting at all. What else besides the Booster being defective could cause this ? ( Vaccuum is at 11 in. w.c. at idle, but has always worked fine in the past.). Thanks.
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If the pedal goes all the way down, check your master cylinder. The booster simply makes it hard or easy to push the pedal.

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Replace the master cly..it has an internal leak...boster is not the problem..
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If the pedal goes all the way down, you have a leak in the brake line somewhere. If you can pump up the brakes, then you have a worn master cylinder and it needs rebuilding. Fluid is leaking around the seals.
Check each wheel to see if the brake cylinders are leaking. A common problem with Vettes that are stored for the winter and you use normal brake fluid is that with the temperature changes, the fluid absorbs moisture. The moisture pits the inside of the cylinders and you lose pressure. The solution is to use silicon brake fluid but a lot of people find it gives a mushy feel to the brakes. The other solution is to replace the cylinders with stainless steel ones.
Check to see if the brake hoses to the wheel cylinders are leaking. Check to see if a brake line has got pinched or ruptured.
Good luck!
Tom in Missouri wrote:

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'If the pedal goes all the way down, you have a leak in the brake line somewhere. If you can pump up the brakes, then you have a worn master cylinder and it needs rebuilding. Fluid is leaking around the seals. Check each wheel to see if the brake cylinders are leaking. A common problem with Vettes that are stored for the winter and you use normal brake fluid is that with the temperature changes, the fluid absorbs moisture. The moisture pits the inside of the cylinders and you lose pressure. The solution is to use silicon brake fluid but a lot of people find it gives a mushy feel to the brakes. The other solution is to replace the cylinders with stainless steel ones. Check to see if the brake hoses to the wheel cylinders are leaking. Check to see if a brake line has got pinched or ruptured. Good luck!'
REPLY: Its going in for diagnosis this friday a.m. to my vette mechanic. He said over the phone that its either the Master Cylinder or possible a caliper . In any case its been ages since the brakes were examined since i drive it 2000 miles max. per year (April thru Oct) . I do have silicone brake fluid in the car . I can pump up the brakes and get them to work good . Ill let you know what the problem is by friday. Thanks for the info and tips. Dave.
Tom in Missouri wrote: If the pedal goes all the way down, check your master cylinder. The booster simply makes it hard or easy to push the pedal.
'Testing the booster is a simple operation of pressing the pedal and seeing if it holds or descends to the floor. It it outlined in all the manuals for the car.'
ME: On my 1970, this is happening all of a sudden ; pedal goes down to nearly floor level. No brake boosting at all. What else besides the Booster being defective could cause this ? ( Vaccuum is at 11 in. w.c. at idle, but has always worked fine in the past.). Thanks.
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Got the brakes diagnosed. Two out of the four calipers have leaking seals (most likely due to my very infrequent use of the car at 2000 miles per year with storage for 5 months). I elected to have all four calipers replaced along with pads. Booster is fine as are the rotors and brake lines, .
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dave wrote:

DILV can't even fix his own brakes? He must like wasting money A booster takes 30 seconds to check for proper operation Did the Xtian maroon have the rotors checked for run-out?
Randy
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'What a maroon '
REPLY : Maroon is a dark red color. Please take the day off tomorrow from stalking me , and, go shopping for a Corvette.
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http://www.nonstick.com/sounds/Bugs_Bunny/ltbb_052.wav
Randy the Rump Ranger
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Might I suggest a power bleeder from Summit Racing www.summitracing.com for about 65.00 or so and then bleed from the right rear then left rear then right front then finish left front. I had the same problem with my 78 and it works great now since last year. I got these intructions from someone in this group ( Dad maybe) but they where correct about the way C-3 Vettes are to be bleed. Best of Luck to you

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