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Old 09-14-2008, 11:23 PM
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What's wrong with my brakes?

Haven't driven my car for a long time as I have been rebuilding the engine/transmission but went to hook up the vacuum hose to the brake booster started the engine and immediately the pedal is pinned to the floor and won't move. I used the big vacuum port at the front of my Quadrajet...by the way this is a blown engine but I have lots of vacuum at idle. When I disconnect the hose the pedal returns to its normal position. I tried bleeding the brakes but that changed nothing. What's wrong?

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Old 09-14-2008, 11:45 PM
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When I was running a Q Jet the brake booster connected to the rear of the carb.
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Old 09-14-2008, 11:58 PM
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Do the brakes work at all? It sounds strange that the pedal would go all the way to the floor, just from the assist in pressure that is provided from the booster, even if the air valve is messed up. . I would guess that you have some brake pedal/pushrod adjustment issues, and your booster air valve is messed up.

(Atmospheric pressure is really doing the work inside your booster, vacuum by itself does nothing.)


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Old 09-15-2008, 12:34 AM
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For some reason the large vacuum port is at the front of my particular Q-jet. I think it's from the early to mid '70s. Anyway, it is providing an adequate vacuum source. I suspect you are right about the air valve inside the booster. It may not be functioning correctly. Guess there's nothing I can do about it accept to replace it.
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Old 09-15-2008, 10:41 AM
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What's wrong with my brakes?

that front port is the pcv port .
in back should be a outlet for brake booster .
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Old 09-16-2008, 12:07 AM
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The back ports look way to small to provide enough vacuum...the hose would have to be a different diameter than the check valve. But even so wouldn't the booster still work if I connected it to the pvc port or is this too much vacuum and somehow overwhelms the internal air valve?
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Old 09-16-2008, 01:00 AM
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Yes the booster will still work fine whether the port is supposed to be for PVC or brake booster.


The PVC will work fine on the front or rear...it all goes under the throttle plate..
Is there a plug in the back of the carb where another vacuum port might be? Or possibly a vacuum port in the manifold? (Many Fords attach the vacuum hose to the booster through a port tapped in one of the intake runners on the manifold)


If there is not, I've put a "T" in the line to run the PVC and booster off the same port. I never saw any difference in how the brakes or PVC valve worked. Lots of Holleys have only one 3/8 vacuum port.


It only takes a few seconds to draw a vacuum in a booster, then the vacuum check valve maintains the vacuum, so for the majority of time, there is minimal flow of air out of the booster. The only reason for a big hose is to facilitate fast recovery of the vacuum in the booster for more consistant pedal effort in stop and go driving.


Later, mikey
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Old 09-16-2008, 08:12 AM
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powersrodsmike , you never had one fail , well don't hit them brakes twice .
that vacuum will let you down , and if your running a bigger cam , even that booster can on the fender well want help .

the front port is to mix with them gas's from the engines pcv valve for better burn i read from gm .
that quadrajet should have a port , plugged maybe ?
or it's a older type or made for pontiac or a olds .
some the older chevy's hooked directly in the intake back of carb location .
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Old 09-16-2008, 10:10 AM
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Quote:
Originally Posted by burnt olds
powersrodsmike , you never had one fail , well don't hit them brakes twice .
that vacuum will let you down , and if your running a bigger cam , even that booster can on the fender well want help .

the front port is to mix with them gas's from the engines pcv valve for better burn i read from gm .
that quadrajet should have a port , plugged maybe ?
or it's a older type or made for pontiac or a olds .
some the older chevy's hooked directly in the intake back of carb location .

Why do you feel the need to confuse this issue with scenarios that do not exist..

The OP has said nothing about having a "big cam",or low vacuum, and as far as we know he only has one port on his carb.

If he only has one port, like most Holleys do, then using a T to get vacuum to his brake booster will present no problems.

You are right about the gases mixing better when they go into the front of the carb, but as the function of the booster will not change the function of the PVC system enough to make any difference.


Quote:
Originally Posted by burnts olds
powersrodsmike , you never had one fail , well don't hit them brakes twice .

You act as if I am guessing about this, I am not guessing, I have plumbed many cars with a T into the PVC line, even cars equipped with a blowers with no problems. (blown cars make alot of vacuum at the baseplate of the carb, as long as the carb is on top of the blower ) My own Blazer, with a moderate cam, has had a T'd line running to the PVC and booster for over 100000 miles with no loss in brakes, even when towing a loaded car trailer, and the motor is doing fine.


If what you "read", differs from my experience, then that does not make my experience wrong.

Many holleys come with 1 big vacuum port, used quite regularly to serve both the PVC and brake booster.


The ability for a running motor to create a vacuum through any port in the manifold is the same, as long as the channels from the port are ported to someplace under the throttle plates, and the channels are large enough to flow properly.


A 3/8" port on a carb is is ported to plenty large enough channels.

Beyond the point when the vacuum is drawn down in the booster, it requires no constant source of vacuum, as long as the diaphram is intact and the check valve is functioning properly. This allows the pvc valve to function as it should .The only time that booster needs to have an increased flow of vacuum is when it is activated. Typically when the booster is activated, the throttle plates are closed, the motor is decelerating and the vacuum is the highest, and the need for the pvc valve to allow crankcase gases to enter the intake are the lowest..


If, as you have speculated,
Quote:
that vacuum will let you down
,then the OEMs would not have designed millions of cars with a vacuum booster to assist in braking.


If a long cam/low engine vacuum or so much blowby that the pvc valve is overwhelmed is a problem, that is a whole seperate issue, and no matter where the vacuum gets hooked to on the carb or manifold it won't let the booster work properly.


If the OP plumbed a vacuum line into the manifold with a supercharged motor, he'd really be asking for trouble.

( I was amiss when I asked if he had a port on the manifold, my initial reading of his post missed the fact that he has a blower. My mistake. )

If you understand the way those systems work, instead of trying to keep everything hooked up the way the factory did it, you would understand that there is no problem..


Later, mikey.
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Last edited by powerrodsmike; 09-16-2008 at 10:58 AM.
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Old 09-16-2008, 11:06 AM
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What's wrong with my brakes?

power rods mike , you didn't read the guy's quote ,carb is as he said quadrajet .
again i say you haven't had it happen till you do .
i've been there , experienced it , not just running my mouth as you getting tore up about it saying things back to argue your point .
i'm trying to help and as normal someone gets bent out of shape .
calm down , i'm sorry you can't read .
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Old 09-16-2008, 11:30 AM
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Quote:
Originally Posted by burnt olds
power rods mike , you didn't read the guy's quote ,carb is as he said quadrajet .
again i say you haven't had it happen till you do .
i've been there , experienced it , not just running my mouth as you getting tore up about it saying things back to argue your point .
i'm trying to help and as normal someone gets bent out of shape .
calm down , i'm sorry you can't read .


You stating your assumptions about my reading ability and attitude are going to direct you into another suspension. If you wish to look at the whole picture, instead of focusing on just one aspect of the problem, then we can continue this discussion.

Mikey
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Old 09-16-2008, 01:03 PM
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What's wrong with my brakes?

thats the way it goes here , you try to help some but you piss off people who can't be wrong , or even admit to it every now and then .
as i said , i'm sorry .
moderator's , you don't know till it happens , right ?
we'll it happened to me .thats all i stated .
it's a quadrajet carb , is that not what the guy said he has ?
true it's hard to believe when your wrong , so you push the effect your a moderator and it's your way or suspension for others .

you show me one gm car that came with the brake vacuum and the pcv valve hooked together , and not show your moderating abilities to rasp out suspension .
is this between you and me now and not about helping the other guy out ?
you seem you want a piece of me , not to help .
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Old 09-16-2008, 02:12 PM
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There's a boatload of misinformation in this thread. First, the brake booster neither knows nor cares where it gets it's vacuum from. All it cares is that the vacuum is applied through the port on the front of the booster. Second, as correctly noted by powerrodsmike, the vac signal is the same whether you use the PCV port or the booster port on the carb/manifold. None of this is the cause of the problem.

A vacuum brake booster has a valve on the pedal side of the housing (concentric with the pushrod) that lets air into the backside of the diaphragm when you apply the pedal. This valve should only open when you press on the pedal and should open in proportion to the force on the pedal. What's happened is that this valve is stuck open. There are several reasons why this can happen. The result is that air is admitted on the pedal side of the booster diaphragm, allowing the vacuum on the M/C side of the diaphragm to suck the pedal to the floor. Bottom line is that the booster is bad and needs to be rebuilt or replaced.
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Old 09-16-2008, 02:20 PM
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Quote:
Originally Posted by joe_padavano
There's a boatload of misinformation in this thread. First, the brake booster neither knows nor cares where it gets it's vacuum from. All it cares is that the vacuum is applied through the port on the front of the booster. Second, as correctly noted by powerrodsmike, the vac signal is the same whether you use the PCV port or the booster port on the carb/manifold. None of this is the cause of the problem.

A vacuum brake booster has a valve on the pedal side of the housing (concentric with the pushrod) that lets air into the backside of the diaphragm when you apply the pedal. This valve should only open when you press on the pedal and should open in proportion to the force on the pedal. What's happened is that this valve is stuck open. There are several reasons why this can happen. The result is that air is admitted on the pedal side of the booster diaphragm, allowing the vacuum on the M/C side of the diaphragm to suck the pedal to the floor. Bottom line is that the booster is bad and needs to be rebuilt or replaced.
Exactly right...I agree. Replace the booster and you should be good to go.
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Old 09-16-2008, 02:50 PM
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Quote:
Originally Posted by burnt olds
thats the way it goes here , you try to help some but you piss off people who can't be wrong , or even admit to it every now and then .
as i said , i'm sorry .
moderator's , you don't know till it happens , right ?
we'll it happened to me .thats all i stated .
it's a quadrajet carb , is that not what the guy said he has ?
true it's hard to believe when your wrong , so you push the effect your a moderator and it's your way or suspension for others .

you show me one gm car that came with the brake vacuum and the pcv valve hooked together , and not show your moderating abilities to rasp out suspension .
is this between you and me now and not about helping the other guy out ?
you seem you want a piece of me , not to help .
Burnt olds-


I would not care about your incorrect information, other than to correct you, if you had not belittled my person, and stayed focused on the technical aspect of this problem...


It is a quadrajet on a motor WITH A BLOWER...

So your stock application references are not applicable..


Show me one GM car stock equipped with a blower and a quadrajet.

The fact that I'm a moderator here does not diminish my knowledge about the automotive systems that I write about, in fact , my ability to not attack a person who is wrong, while debating the technical aspect of the problem, is one of the reasons why I am a mod now.

Historically you seem to have nothing but disdain for the mods and administration here, we all get along just fine with the majority of members here.

Argue the tech, understand the system without belittling the person who dissagrees with you and you'll have no problem.

Continue writing personal attacks and I will be happy to give you a suspension.

You refuse to acknowledge any part of this guys problem except the fact that he has a quadrajet... I even explained how the systems work together, so that everyone, including YOU may get a better understanding them, but you ignore all of that to pick a fight with me, for reasons that only you know.


The ability to help a person with an automotive technical problem requires understanding the system, how it works and WHY it works... not just remembering which hose went where.


You can believe that you are right, that is your right...but correcting your disinformation is my right and chosen responsibility , as a member here.


Joe Padavano, thank you, if you will see my first response to the op I said he probably had a bad booster air valve, and possibly other issues.



Later, mikey
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