Brake problem
Thread Starter
Heel & Toe
Joined: Nov 2005
Posts: 23
Likes: 0
Brake problem
Well, I thought I had the brakes licked on my 74 L48 with standard brakes. Last night while driving, the pedal sunk and I lost a great deal of brake pressure...not all, but a significant amount. The brake light also went on. I checked the master cylinder and both front and rear chambers are full. I do not see a leak anywhere.
Prior to this, I have
Replaced front calipers and pads
Replaced all brake lines
Replaced a leaky proportioning valve
Bled brakes using Motive power bleeder
I'm wondering what to do at this point. I think that I may have air in the lines and that I may need to rebleed or that the master cylinder needs replacing. I just don't know why the brake light would go on with a full master cylinder. Any help and advice would be greatly appreciated. Thanks.
Prior to this, I have
Replaced front calipers and pads
Replaced all brake lines
Replaced a leaky proportioning valve
Bled brakes using Motive power bleeder
I'm wondering what to do at this point. I think that I may have air in the lines and that I may need to rebleed or that the master cylinder needs replacing. I just don't know why the brake light would go on with a full master cylinder. Any help and advice would be greatly appreciated. Thanks.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Originally Posted by jberman
Any help and advice would be greatly appreciated. Thanks.
Before you buy or rebuild anything, re-bleed. Corvettes had a very specific bleed order in mind when they designed the system. I would suggest doing it the old fassioned way this time, and try this order.
Driver's Rear
Passenger's Rear
Passenger's Front
Driver's Front
At least five Pump/bleeds per side.
If after a good bleed you still have a mushy pedal, it wouldn't hurt to rebuild the master. It only costs $25 for the kit, and take about an hour.
In the meantime, here's a test for you to check the booster.
With the engine off, pump the pedal until it gets hard. If it doesn't get hard you have a bad booster.
Once the pedal is hard, start the engine. The pedal should go soft and go most of the way to the floor if you have air in the lines.
If it gets hard with the engine off, and soft when you start it...your booster is most likely okay. It's a good idea to rebuild that master, but only if a bleed doesn't help.
Also, inspect ALL wheels for ANY signs of fluid loss. If you don't already have stainless steel sleeved pistons in the calipers you may be getting air in the system via a bad seal or runout....also known as rotor wobble.
Instructor
Joined: Sep 2005
Posts: 241
Likes: 0
From: St Louis Missouri
Are the brakes always soft, or only once or twice? I have a 75 and after replacing the calipers and lines it took hours to bleed it all. I was using the old wife b!tching at me 3000 to get it done, so it took a while. After that the pedal is solid with or without the engine on for the vacuum boost.
If it happens all the time I would suspect an air bubble, but if it’s intermittent you may have to look into the master cylinder or vacuum assist.
If it happens all the time I would suspect an air bubble, but if it’s intermittent you may have to look into the master cylinder or vacuum assist.
Thread Starter
Heel & Toe
Joined: Nov 2005
Posts: 23
Likes: 0
Originally Posted by Durango_boy
Get a newer model with less miles. 
J/K
J/K
Le Mans Master
Joined: Jan 2006
Posts: 9,192
Likes: 27
From: Boca Raton Florida
The reason the light came on is because the proportioning valve shifter, indicating a failure in one side of the brake system. It did what it was supposed to do. You need to do a couple things to be done with this issue:
1. check rear rotors for SS sleeves
2. Measure runout on all 4, less than .002"
3. new seals on the rear calipers
4. Rebleed
You can get your pedal back for noww by rebleeding but that won't fix the problem, I would personally go with the o-ring conversion kit on all 4.
1. check rear rotors for SS sleeves
2. Measure runout on all 4, less than .002"
3. new seals on the rear calipers
4. Rebleed
You can get your pedal back for noww by rebleeding but that won't fix the problem, I would personally go with the o-ring conversion kit on all 4.
Thread Starter
Heel & Toe
Joined: Nov 2005
Posts: 23
Likes: 0
Originally Posted by SIXFOOTER
The reason the light came on is because the proportioning valve shifter, indicating a failure in one side of the brake system. It did what it was supposed to do. You need to do a couple things to be done with this issue:
1. check rear rotors for SS sleeves
2. Measure runout on all 4, less than .002"
3. new seals on the rear calipers
4. Rebleed
You can get your pedal back for noww by rebleeding but that won't fix the problem, I would personally go with the o-ring conversion kit on all 4.
1. check rear rotors for SS sleeves
2. Measure runout on all 4, less than .002"
3. new seals on the rear calipers
4. Rebleed
You can get your pedal back for noww by rebleeding but that won't fix the problem, I would personally go with the o-ring conversion kit on all 4.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
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
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Originally Posted by jberman
Will do...how do you tell if the calipers have the sleeves?
Do like Mark Hamill would...use the Force.
Seriously, you'll have to remove the caliper, and take out the pads. Peel back the seal, and you shoud see a prominent edge where the sleeve starts.
Another way is past knowledge of the brake work done. If the rear brakes have never been changed then they are NOT sleeved.
Melting Slicks
Joined: Sep 2002
Posts: 2,390
Likes: 2
From: New Castle DE
just have seen this listed before on brake bleeding threads. apparently if you go to the front ones then the back there is a chance air can get trapped in between. I just bled it this way and it worked out fine but i haven't done it differently so can't say if it would make a difference.
Melting Slicks
Joined: Sep 2002
Posts: 2,390
Likes: 2
From: New Castle DE
Originally Posted by Durango_boy
Like when I said this?
actually i thought it was
passenger rear (inside bleeder then outside)
driver rear (inside bleeder then outside)
passenger front
driver front
i didn't notice that in your first post. I always read you go from the caliper farthest from the master cylinder and work your way back.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Originally Posted by IMA66/74
actually i thought it was
passenger rear (inside bleeder then outside)
driver rear (inside bleeder then outside)
passenger front
driver front
i didn't notice that in your first post. I always read you go from the caliper farthest from the master cylinder and work your way back.
passenger rear (inside bleeder then outside)
driver rear (inside bleeder then outside)
passenger front
driver front
i didn't notice that in your first post. I always read you go from the caliper farthest from the master cylinder and work your way back.
That's how you get air trapped in the system. The manual suggests couter clockwise from the left rear.
Thread Starter
Heel & Toe
Joined: Nov 2005
Posts: 23
Likes: 0
Originally Posted by Durango_boy
That's how you get air trapped in the system. The manual suggests couter clockwise from the left rear.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Originally Posted by jberman
I also find it puzzling why I can't get significant pressure by pumping the brake pedal while the car is off. I'm not giving up!!!
Whoa...this is a warnign sign. Did you say that whent he engine is off, you pump the pedal and it never gets hard? That's a warning sign that your booster's rubber diaphram is not holding the pressure, and may be leaking. You may be looking at getting a new booster man.
Team Owner
Joined: Oct 2004
Posts: 23,831
Likes: 10
From: Springfield MO
St. Jude Donor '07
I could have sworn that the manual says to use the method described above, with the inside bleeders on the rear, then the outside. Corvettes have a front/back twin circuit, not the "X" pattern seen on some cars.
I'll have to check that when I get home. I haven't bled my brakes for a while.
I'll have to check that when I get home. I haven't bled my brakes for a while.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Originally Posted by I'm Batman
I could have sworn that the manual says to use the method described above, with the inside bleeders on the rear, then the outside. Corvettes have a front/back twin circuit, not the "X" pattern seen on some cars.
I'll have to check that when I get home. I haven't bled my brakes for a while.
I'll have to check that when I get home. I haven't bled my brakes for a while.
Yeah, it's rears first...you're right, but you start on the driver's side, switch sides, then move forward on the passenger side without X-ing over. Do the front passenger side third, and then cross back over to driver's side for the last bleed in the front.
You're corrct that it's not 'X' pattern.
Thread Starter
Heel & Toe
Joined: Nov 2005
Posts: 23
Likes: 0
Originally Posted by Durango_boy
Whoa...this is a warnign sign. Did you say that whent he engine is off, you pump the pedal and it never gets hard? That's a warning sign that your booster's rubber diaphram is not holding the pressure, and may be leaking. You may be looking at getting a new booster man.
