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'77 L48 brake help

 
Old 05-20-2019, 07:41 PM
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amvenne
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Default '77 L48 brake help

Hey guys, I'm new to the vette scene. I've been in love with the C3 for a long time and finally pulled the trigger on one. Love the forum and all the help everyone is providing. I'm sure I'll be pinging the crowd a lot for help. First question: my brakes aren't currently working. I only get stopping power when the brake is fully depressed, and even then its only some of the time. I bled all of the lines and it didn't help. I traced all of the lines to the calipers and didnt see any leaks. Where should I look next? MC? Power booster?

TV
1977 L48
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Old 05-20-2019, 08:10 PM
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C3's are infamous for air in the brake lines and for being hard to bleed. I would try bleeding more. Do you get a better pedal if you pump the brakes a few times and then hold it down?
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Old 05-20-2019, 08:46 PM
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I would bet somebody elses money, that somewhere along the lines of past history that somebody let the master go dry, added fluid and called it good. Letting it run dry introduced air into the system.
But before you start bleeding all six bleeders, I would remove the steel lines from the MC and plug them with a suitable plug, not a bolt. Then, if you can stand on the brake pedal, you have eliminated that portion as a suspect. If the pedal goes to the floor, then you know what part to order next.

If the MC is good, your issue lies elsewhere. Then reattach the lines and bleed all the calipers with fresh fluid. Check all the connections at the Proportioning Valve and any distribution blocks for wetness.

And the booster is strictly an assist. It allows you to lock up all four, barefoot if you want. It has little effect on a pedal to the floor. Especially with the engine off. You should still have "manual" brakes regardless.

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Old 05-20-2019, 09:01 PM
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Yup, bleeding is not easy on these. The second time I bled the 79, after leaving a bit of air in the lines the first time (same symptoms), I made sure to do the following:

Used a Motive Pressure Bleeder (I justed used it for air pressure, and kept the MC full, periodically checking and refilling, NEVER letting it run dry).
Used Motive bleeder bottles.
Tapped on the lines, junctions, and calipers with a small wrench, in the hopes of freeing trapped air bubbles
Used a lot of brake fluid.

I did not pump the brakes, but if you do it that way, you may want to consider some thread sealant on the bleeder valves (threads ONLY, not the tapered face seal) to keep them from sucking air back in.
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Old 05-20-2019, 09:22 PM
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Looks like I'll be trying more bleeding. I was going to take the MC off, bench bleed it, and then bleed the lines again.... but one of the bolts from the MC to the booster is practically welded on. I've got a ton of leverage and PB blaster but that sucker isnt moving!
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Old 05-20-2019, 09:27 PM
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they have a pump for bleeding brakes . works really good
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Old 05-21-2019, 08:46 PM
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Originally Posted by 57russ View Post
they have a pump for bleeding brakes . works really good
Who has a pump?
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Old 05-21-2019, 08:50 PM
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autozone our auto parts store. its a small hand held pump , hooks to the bleeder valve and you hand pump untill looks like
good stream
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Old 05-22-2019, 12:30 PM
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I'm going to check the MC using the method HeadsU.P. described (he's a fellow Michigan guy so I cant go wrong!). Before I start randomly searching for the right size, does anyone know what size plug would plug the holes that feed the lines? Thanks!
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Old 05-22-2019, 01:18 PM
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The pump mentioned above is a hand-held vacuum device that sucks the juice out of the calipers. They work really well providing the bleeder screws fit tightly against their threads. Otherwise the continuous bubbles in the clear hose will drive you crazy.

Another type of pump is the Motive Pressure Bleeder. It works better than vacuum pumps because it has an almost endless supply of brake fluid in its tank. (It looks like a garden sprayer) Look for a used one on eBay. But these types require a custom lid to fit the MC w/o leaking all over the place. I made my own lid and it can be seen in my profile photo album.

As far as plugging the MC ports for a pedal pressure test, I went to NAPA and bought two steel lines about a foot long with brass fittings on the end and flared. By the way, the front MC port has a different diameter and thread pitch than the rear port. Off hand, I do not have those measurements. On the other end of my "MC test lines and just pinched them off with a 180* bend. Do not insert a hardware bolt into the MC port. There is a flare inside that can be damaged.

And Help! Does anybody know the diameter and TPI of the two brake line ports of a C3 master cylinder?

Last edited by HeadsU.P.; 05-22-2019 at 01:21 PM.
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Old 05-22-2019, 04:45 PM
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Well, now I can't get the rear brake line to screw back in to the MC. The front one goes on easy, the rear one wont grab the threads...
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Old 05-22-2019, 05:07 PM
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Originally Posted by amvenne View Post
Well, now I can't get the rear brake line to screw back in to the MC. The front one goes on easy, the rear one wont grab the threads...
I had this problem with a stainless steel fuel line, which is why I'll never buy stainless lines again.

To get around it, I had to make sure the line was perfectly square to the threaded opening, and that the tapered end of the line was seated on the taper at the bottom of the hole. Then I was able to engage the threads. I had to bend the line several times to get everything aligned at the sweet spot.
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Old 05-22-2019, 07:45 PM
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Don't panic. Common issue. What ever you do, don't cross-thread it. I think you are forcing the flared line in too soon and hindering the fitting threads from starting.
Keep the flare flush with the fitting, jiggle, cuss twice, try again, take your time.
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Old 05-22-2019, 09:59 PM
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Originally Posted by HeadsU.P. View Post
Another type of pump is the Motive Pressure Bleeder. It works better than vacuum pumps because it has an almost endless supply of brake fluid in its tank. (It looks like a garden sprayer) Look for a used one on eBay. But these types require a custom lid to fit the MC w/o leaking all over the place. I made my own lid and it can be seen in my profile photo album.
I have a Motive Pressure bleeder that I bought new and it came with 3 or 4 different lids for master cylinders and one works great on my 76. Also, I use the tank for pressure only to avoid any potential mess but I have to keep check on the brake fluid not to let it run dry during pressure bleeding.
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Old 05-22-2019, 10:08 PM
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Originally Posted by ykf7b0 View Post
I have a Motive Pressure bleeder that I bought new and it came with 3 or 4 different lids for master cylinders and one works great on my 76. Also, I use the tank for pressure only to avoid any potential mess but I have to keep check on the brake fluid not to let it run dry during pressure bleeding.
That's just how I do it, too. The Motive works great. I have the cover on and off the MC many times during the operation to check and refill, so I use a sturdy bar clamp to hold the cover on.
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Old 05-22-2019, 10:26 PM
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Finally got the brake lines hooked back up, bled the lines thoroughly.... and still nothing. There is barely any resistance to pushing the brake to the floor. Even after pumping the brakes multiple times prior to starting. I noticed tonight the brake light comes on when I reach the floor with the brake, and after a while it stays on. Any more ideas? I'm tempted to replace the power booster and master cylinder since they are both old and my only thought is that one of those isn't sealing properly?
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Old 05-22-2019, 10:53 PM
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Read the thread I started on this forum entitled BLEEDING WILWOOD CALIPERS to get more insight as to what is involved in bleeding these C3 master cylinders and calipers.
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Old 05-23-2019, 11:50 PM
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Well I think I figured it out. Where the rear brake line meets the MC was a definite leak. (same place I had trouble re-threading). I think there was an issue with the flanges of the brake line and the flange inside the MC, possibly over-tightened previously. I'm going to replace the line and MC and see if that fixes the problem.
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Old 05-24-2019, 12:29 AM
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Could someone help me find the brakeline that runs from the MC to the proportioning valve? I just want to make sure I get the right one. I only need the rear brake line, but if they are sold in a pair, that would be great too. Thanks!
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Old 05-24-2019, 07:28 AM
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Originally Posted by amvenne View Post
Could someone help me find the brakeline that runs from the MC to the proportioning valve? I just want to make sure I get the right one. I only need the rear brake line, but if they are sold in a pair, that would be great too. Thanks!
It should be one of these pairs, depending on your car's options, and steel vs SS:
https://www.zip-corvette.com/68-82-c...year/1977.html

I'm sure the other vendors carry them, too, but this was the fastest I could find them.

Last edited by Bikespace; 05-24-2019 at 07:43 AM.
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