Vacuum for Brakes
Thread Starter
Instructor
Joined: Nov 2007
Posts: 163
Likes: 0
From: Sugarland Texas
Vacuum for Brakes
Have a 69 with aftermarket intake, cam and 750 Holly. Anyone know how much vacuum I should be pulling on the line that goes to the power brake booster. Having to push pedal to far down. Breaking is weak.
Instructor
Joined: Aug 2003
Posts: 104
Likes: 0
From: Newfane NY
Full manifold vacuum should be present... Any Mods to eng???,New problem???
From the Booster back, the line should be the same size as the fitting there, no smaller and usually is fit to the base of the carb...
(black hose to metal pipe to carb)
From the Booster back, the line should be the same size as the fitting there, no smaller and usually is fit to the base of the carb...
(black hose to metal pipe to carb)
Pro
Joined: Jun 2004
Posts: 700
Likes: 0
From: Lantana TX
oem spec call out for 16 but I had mine working fine at 13, but recent new engine producing 8.5 at iddle and they have suffered....installing hydroboost to fix the issue.
Le Mans Master
Joined: Dec 2007
Posts: 6,422
Likes: 591
From: McHenry Illinois
If your pedal is going too far down and the braking is weak, then the booster is probably not the problem. (posibly air in the system) If you need to push harder on the pedal to get good stopping power and the pedal is high, then I would suspect the booster. You can check it by pushing on the brake with the engine off and hold it down while you start it. If the pedal drops slightly after the engine starts, then the booster is working.
Instructor
Joined: Aug 2003
Posts: 104
Likes: 0
From: Newfane NY
All good points... 
Do the brakes work fine all but at idle?
A vacuum booster is a storage device for the brakes and usually will, if not leaking, work fine. In the booster is a check valve that makes this happen.
Most often, modified engines have lower vacuum at idle, and together with a weak booster, will have issues.
Best to make sure as pointed out earlier, that base brake problems are not an issue including rotor run-out, air in hydraulic system, leaking booster.. and check valve.
To test, after running the car, shut it off and press the pedal up to 5 times or untill the pedal feels firm and hard to push.
If no difference, possible booster or air in hydralic system.
Pedal travel should not be excessive.
Do the brakes work fine all but at idle?
A vacuum booster is a storage device for the brakes and usually will, if not leaking, work fine. In the booster is a check valve that makes this happen.
Most often, modified engines have lower vacuum at idle, and together with a weak booster, will have issues.
Best to make sure as pointed out earlier, that base brake problems are not an issue including rotor run-out, air in hydraulic system, leaking booster.. and check valve.
To test, after running the car, shut it off and press the pedal up to 5 times or untill the pedal feels firm and hard to push.
If no difference, possible booster or air in hydralic system.
Pedal travel should not be excessive.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
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
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
Burning Brakes
Joined: Jan 2009
Posts: 1,224
Likes: 1
From: Clarksville Indiana
Its been my experiance that your pedal will get rock hard and not stop well when there is a vacuum problem. For a low mushy pedal I would bleed the brakes good before I replaced the master cylinder.
Melting Slicks
Joined: Sep 2007
Posts: 2,429
Likes: 0
From: Tucson AZ
I don't know what you've done so far, but........
I replaced all the vacuum lines for the brakes, headlights, and wiper system. Still had low vacuum, and mushy brakes. Replace the master cylinder and the brake booster. Alot better, the booster was leaking internal. I am in the middle of a heater core & A/C evaperator core fix, and have replaced all the vacuum lines for that as well. As soon as I'm done with that, I'm going to rebuild all the calipers. I'll have a good clean brake system when I'm done, and it will be even better than it was the last time I drove it, which was pretty good, by the way.
Good luck, let us know,
Tank
I replaced all the vacuum lines for the brakes, headlights, and wiper system. Still had low vacuum, and mushy brakes. Replace the master cylinder and the brake booster. Alot better, the booster was leaking internal. I am in the middle of a heater core & A/C evaperator core fix, and have replaced all the vacuum lines for that as well. As soon as I'm done with that, I'm going to rebuild all the calipers. I'll have a good clean brake system when I'm done, and it will be even better than it was the last time I drove it, which was pretty good, by the way.
Good luck, let us know,
Tank
Racer
Joined: Dec 2003
Posts: 487
Likes: 160
From: Two Lane Black Top
The master cylinder is easy to replace the brake booster is VERY hard. Had to have help from two friends to do that.
