Poor braking at idle
Thread Starter
7th Gear
Joined: Jun 2024
Posts: 7
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Poor braking at idle
Hi,
I have a bit of a strange issue. My whole brake system from end to end is brand new and under normal driving the brakes work great. However if I’m idling or driving at very low speed like parking of rolling up to junctions the pedal is very hard and braking is poor. Has anyone seen this before and can anyone offer any advice to troubleshoot it? Like i say driving normally the brakes are perfect
thanks
I have a bit of a strange issue. My whole brake system from end to end is brand new and under normal driving the brakes work great. However if I’m idling or driving at very low speed like parking of rolling up to junctions the pedal is very hard and braking is poor. Has anyone seen this before and can anyone offer any advice to troubleshoot it? Like i say driving normally the brakes are perfect
thanks
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
How big of a cam are you running, what's it's LSA spec, and how much vacuum is your engine pulling at idle? Timing will also have a huge effect on engine vacuum - what is your timing set up to, and are you running vacuum advance off manifold vacuum to produce actual timing at idle in the high 20's or close to 30?
Also, your profile states nothing about you or your car - you might want to fill that out so we know what you're talking about...
Also, your profile states nothing about you or your car - you might want to fill that out so we know what you're talking about...
Thread Starter
7th Gear
Joined: Jun 2024
Posts: 7
Likes: 0
Thread Starter
7th Gear
Joined: Jun 2024
Posts: 7
Likes: 0
How big of a cam are you running, what's it's LSA spec, and how much vacuum is your engine pulling at idle? Timing will also have a huge effect on engine vacuum - what is your timing set up to, and are you running vacuum advance off manifold vacuum to produce actual timing at idle in the high 20's or close to 30?
Also, your profile states nothing about you or your car - you might want to fill that out so we know what you're talking about...
Also, your profile states nothing about you or your car - you might want to fill that out so we know what you're talking about...
Drifting
Joined: Jul 2022
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From: Wisconsin
2024 C3 of the Year Finalist- Modified
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Thread Starter
7th Gear
Joined: Jun 2024
Posts: 7
Likes: 0
How big of a cam are you running, what's it's LSA spec, and how much vacuum is your engine pulling at idle? Timing will also have a huge effect on engine vacuum - what is your timing set up to, and are you running vacuum advance off manifold vacuum to produce actual timing at idle in the high 20's or close to 30?
Also, your profile states nothing about you or your car - you might want to fill that out so we know what you're talking about...
Also, your profile states nothing about you or your car - you might want to fill that out so we know what you're talking about...
Is my vette stock?? HAHA
Joined: Oct 2015
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
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Is your engine cammed? Do you still have vacuum headlight doors?. You may have a good sized leak that is overcome with engine speed. I would pull a vacuum on you vacuum booster and see if it holds it. The valve in the booster could be bad as well
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Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
Thread Starter
7th Gear
Joined: Jun 2024
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Race Director
Joined: Mar 2017
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From: Virginia
Post some photos with the air cleaner removed!
Do you have a way to measure vacuum? Is your brake booster connected to a dedicated port on the intake manifold, or is something else going on?
Do you have a way to measure vacuum? Is your brake booster connected to a dedicated port on the intake manifold, or is something else going on?
Thread Starter
7th Gear
Joined: Jun 2024
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i will get something to measure the vacuum and report back. Thanks!
Drifting
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From: Wisconsin
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Melting Slicks
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
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Race Director
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From: Virginia
Low hanging fruit:
There is a check valve where the vacuum "enters" the brake booster. You should have two or three good brake pumps even if your engine stalls. If the check valve is doing it's job, you might not notice the low vacuum at idle. You can test this with a Mity-Vac (which can also act as a vacuum gauge).
The brake booster should be connected to its own port. Please post some photos without the air cleaner attached, and folks might suggest something. Do you still have the carbon filter that keeps the brake booster from being damaged by fumes?
There is a check valve where the vacuum "enters" the brake booster. You should have two or three good brake pumps even if your engine stalls. If the check valve is doing it's job, you might not notice the low vacuum at idle. You can test this with a Mity-Vac (which can also act as a vacuum gauge).
The brake booster should be connected to its own port. Please post some photos without the air cleaner attached, and folks might suggest something. Do you still have the carbon filter that keeps the brake booster from being damaged by fumes?
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
Boosters don't like to be "T"ed with another hose.
Top Priority! You need a dedicated line from the back of the carb or a vacuum fitting on the Intake to the booster.
It should be labeled as booster hose. Larger diameter than the rest, but more importantly "thick-walled hose" so it won't collapse.
If you need lots of vacuum ports, EBay has NPT fittings that have 2,3 or 4 ports for accessories.
Old Pro Solo Guy
Joined: Dec 2017
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From: Marlton NJ
Lars covered all the possibilities.
Engines definately make more vacuum while rev'd up, and are lowest at idle.
If they have a big cam, it can get very low at idle.
Does it sound like it has a big cam at idle? EX: How steady and smooth is it or does it lope?
Engines definately make more vacuum while rev'd up, and are lowest at idle.
If they have a big cam, it can get very low at idle.
Does it sound like it has a big cam at idle? EX: How steady and smooth is it or does it lope?
Is my vette stock?? HAHA
Joined: Oct 2015
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
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The early cars were all Teed like that. Try disconnecting the other hoses and just having the brake line alone with everything else plugged. Keep the vacuum advance connected and try that. If there is no change increase your idle speed to 900 rpm and try again.
What is your timing set to, if it isn't at 36º with initial and mechanical (all in) with vacuum advance disconnected at 3000 rpm then set that and try again
If it's still bad I would guess you may need a vacuum assist pump. I added one years ago on a small block for this same reason. Some cams just don't produce decent vacuum at idle.
What is your timing set to, if it isn't at 36º with initial and mechanical (all in) with vacuum advance disconnected at 3000 rpm then set that and try again
If it's still bad I would guess you may need a vacuum assist pump. I added one years ago on a small block for this same reason. Some cams just don't produce decent vacuum at idle.