Vacuum to brake booster
Thread Starter
Burning Brakes
Joined: Nov 2006
Posts: 893
Likes: 7
From: Orange Park Florida
Vacuum to brake booster
I've noticed that when the car is idling (in neutral) and I depress the brake pedal, the engine increases rpms - 100 - 200 rpms then settles back to the normal 800 rpm setting. The car is an automatic and the brakes seem to work fine. I have just noticed this, so is this a common engine response or do I have some kind of minor vacuum leak, ie brake booster diaphragm, check valve, or hose. Any comments would be appreciated.
Jim
Jim
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
That sympton would indicate that the vacuum booster wasn't holding the vacuum completely. It does not mean that the booster is 'faulty'...just that the seals are a bit worn. There should be no issue of brake function or safety.
If that condition worsens significantly and you become more concerned, you might want to replace the booster unit.
It might also indicate that your carb is running a bit 'rich' at idle, and the added air from the booster (when the brake is applied)...along with the excess fuel in the [rich] intake charge...causes the engine to rev just a bit.
If that condition worsens significantly and you become more concerned, you might want to replace the booster unit.
It might also indicate that your carb is running a bit 'rich' at idle, and the added air from the booster (when the brake is applied)...along with the excess fuel in the [rich] intake charge...causes the engine to rev just a bit.
Last edited by 7T1vette; Aug 20, 2014 at 12:28 PM.
Race Director
Joined: Feb 2001
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Likes: 2,098
From: Grapevine Tx
2018 C3 of Year Finalist
2016 C3 of the Year Finalist
That sympton would indicate that the vacuum booster wasn't holding the vacuum completely. It does not mean that the booster is 'faulty'...just that the seals are a bit worn. There should be no issue of brake function or safety.
If that condition worsens significantly and you become more concerned, you might want to replace the booster unit.
It might also indicate that your carb is running a bit 'rich' at idle, and the added air from the booster (when the brake is applied)...along with the excess fuel in the [rich] intake charge...causes the engine to rev just a bit.
If that condition worsens significantly and you become more concerned, you might want to replace the booster unit.
It might also indicate that your carb is running a bit 'rich' at idle, and the added air from the booster (when the brake is applied)...along with the excess fuel in the [rich] intake charge...causes the engine to rev just a bit.
Burning Brakes
Joined: Jul 2009
Posts: 1,240
Likes: 294
From: ESCONDIDO CA
2025 C2 of the Year ('63 and '67) Finalist - Unmodified
C2 of Year Finalist (appearance mods) 2019
When I brake hard (while driving) my rpms drop-sometime significantly so that I have to feather in some accelerator to keep the motor running.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
When you apply the brakes with a power-brake system, the vacuum in the servo booster 'works' to assist with the apply force. If the booster is well sealed, there should not be any loss of vacuum or transfer any air between the booster and the intake. But years of use wear seals, and eventually, they will begin to leak a bit. That would allow excess air to enter into the A/F charge in the intake system. And, extra air put into a balanced fuel charge will lean it out.
Had a 1976 L-82, 4-sp
Joined: Mar 2011
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From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
I have a question. When I pinch my vacuum hose between the intake manifold and booster, the engine dies. Is this normal?
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