'77 L48 brake help
Thread Starter
Instructor
Joined: May 2019
Posts: 103
Likes: 13
From: Near D.C.
'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
TV
1977 L48
Le Mans Master
Joined: Dec 2007
Posts: 6,422
Likes: 591
From: McHenry Illinois
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?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
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.
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.
Last edited by HeadsU.P.; May 20, 2019 at 08:52 PM.
Race Director
Joined: Mar 2017
Posts: 11,927
Likes: 4,497
From: Virginia
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.
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.
Thread Starter
Instructor
Joined: May 2019
Posts: 103
Likes: 13
From: Near D.C.
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!
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
Thread Starter
Instructor
Joined: May 2019
Posts: 103
Likes: 13
From: Near D.C.
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!
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
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?
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.; May 22, 2019 at 01:21 PM.
Race Director
Joined: Mar 2017
Posts: 11,927
Likes: 4,497
From: Virginia
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.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
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.
Keep the flare flush with the fitting, jiggle, cuss twice, try again, take your time.
Racer
Joined: Aug 2014
Posts: 479
Likes: 93
From: Tennessee
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.
Race Director
Joined: Mar 2017
Posts: 11,927
Likes: 4,497
From: Virginia
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.
Thread Starter
Instructor
Joined: May 2019
Posts: 103
Likes: 13
From: Near D.C.
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?
Thread Starter
Instructor
Joined: May 2019
Posts: 103
Likes: 13
From: Near D.C.
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.
Thread Starter
Instructor
Joined: May 2019
Posts: 103
Likes: 13
From: Near D.C.
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!
Race Director
Joined: Mar 2017
Posts: 11,927
Likes: 4,497
From: Virginia
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; May 24, 2019 at 07:43 AM.