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Brake booster and PCV hose location?

 
Old 04-19-2003, 12:03 PM
  #1  
tzavaleta
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Default Brake booster and PCV hose location?

The brake booster and the PCV valve (from what i understand) plug into the back of the carb. Is it ok to T these two together? Or will that hamper the effectiveness of the vacuum going into the brake booster?

If i should not "T" these. Then where should i run the PCV? I'd rather not run it into the carb base (unless that is really just the best thing to do). Or i could just run a second breather.

Thanks,
-thomas
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Old 04-19-2003, 03:16 PM
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Paul L
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Default Re: Brake booster and PCV hose location? (tzavaleta)

The brake booster runs into the back of the carb and is quite separate from the PCV hose that runs into the front of the carb.

Here they are.




[Modified by paul79, 3:35 PM 4/19/2003]
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Old 04-19-2003, 04:05 PM
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Default Re: Brake booster and PCV hose location? (paul79)

It really depends what carb. you have ?

If it's a Q-Jet, then the picture above applies. If it's a Holley, and you only have one outlet, then there are a couple options.
Some people "T" the line, but I don't like that idea.

One person ran a large vacuum line from the intake manifold to the power brakes and let the PCV go to the carb, or you could do it the other way around.
Another solution, if you have hood clearance is to get a spacer with a vacuum port built in.
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Old 04-19-2003, 05:38 PM
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Default Re: Brake booster and PCV hose location? (tzavaleta)

Since the PCV valve recirculates crankcase contaminents into the combustion chamber, I don't see how running a T with the brake booster is going to achieve this and sounds a bit risky.

You could run breather valves off the covers, but the contaminents will exhaust into the engine compartment in vapor and liquid form.

The PCV system has been around since 1925 and is designed to protect your motor. Since you've got a big block, you should have room to run a spacer as suggested above, or at worst, swap your carb for one with the right ports.
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Old 04-19-2003, 09:50 PM
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Default Re: Brake booster and PCV hose location? (standup)

Wow there's a rock and a hard place.

My carb is a demon and it only has 1 port.
Since it's a big block i have VERY little room... I was going to ask that question too. I've got a single plain intake but the demon is tall and it sticks up pretty high. In '74 it's the same hood, so i may have to buy an L88 hood(anyone looking to sell one?).

I figured since the line is vacuum it's 'pulling' from both sources (brake boster and PCV). Therefore nothing would be going back into the brake booster from the PCV. Is this an incorrect line of thinking?

-thomas
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Old 04-20-2003, 02:31 AM
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Default Re: Brake booster and PCV hose location? (tzavaleta)

No offense, but I believe this is incorrect thinking.

PCV is an emission control system and works by drawing blow by gases (contaminents) from the crankcase, through the PCV valve and into the combustion chamber to be reburnt.

The brake booster draws vacuum to be stored in a reservoir for proper operation of power brakes.

The two systems are independant and shouldn't be combined, for two reasons:

1. Contaminents may be drawn into the brake booster, thus affecting your brakes.
2. An added T-junction will increase the risk of vacuum leak, thus affecting your brakes.

Your car will still run without a properly functioning PCV system, but your engine will accumulate water, acids and unburned fuel, which means quicker contamination of your oil (and more frequent oil changes) and increased wear and damage on the engine.

The PCV system is also linked to the fuel vapour cannister, which removes fuel vapour pressure from your tank to be reburnt in the combustion chamber. Severing this link means you have to use a vented fuel cap to relieve pressure at the tank or use the cannister to vent fuel vapour: both of which have their own disadvantages.

You don't have the ports because Demon (and other aftermarket carbs) are designed for racing applications, where the PCV and Fuel Vapour systems are not wanted nor required.

The real question is: How big is the risk in modifying your factory system?

1. If you "T" the PCV and brake booster, your brakes may fail.
2. If you use breathers instead of a PCV system, you may have to clean your engine bay more often, you're polluting the atmosphere, you'll have to vent fuel vapour through the fuel cap or cannister, which could create a source of ignition.
3. If you spend $20-40 on a carb spacer with a PCV port, it may not fit under the '74 rear induction hood. (BTW--The rear induction hood on your '74 isn't that much smaller than an L-88).

The idea is to run the PCV system as close to factory as possible, not because of NCRS, but because you don't want to invent a new set of problems.

I hope this clarifies the matter....and good luck!


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Old 04-20-2003, 11:23 AM
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Default Re: Brake booster and PCV hose location? (tzavaleta)

Wow there's a rock and a hard place.

My carb is a demon and it only has 1 port.
Since it's a big block i have VERY little room... I was going to ask that question too. I've got a single plain intake but the demon is tall and it sticks up pretty high. In '74 it's the same hood, so i may have to buy an L88 hood(anyone looking to sell one?).

I figured since the line is vacuum it's 'pulling' from both sources (brake boster and PCV). Therefore nothing would be going back into the brake booster from the PCV. Is this an incorrect line of thinking?

-thomas
Before you go and replace your hood one question - does your single plane intake have an open or divided plenum. If open - you can drill a hole in the side of the plenum near the Carb base, this hole would then be tapped (probably a 3/8-16 fitting) and your PCV can be attached to that - the PCV fitting is a straight vacuum connection - it isnt ported vacuum or anything of that sort. This will allow you to run the PCV separate from teh brake booster which I agree is a good idea. If you have to run a T in that line then add an additional filter to the brake booster line. It wont be pretty but might offer a little more protection.

If you are very careful you should be able to drill that hole without removing the intake - remove the carburator and fill the plenum with shop rags. apply a generous dab of axle grease to the end of the drill bit and go slowly, the grease will hold the aluminum shavings to the drillbit for the most part keeping the majority of them out of the intake. the rags should stop the rest. apply the same grease to the tap you use to tap the hole.
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Old 04-20-2003, 01:22 PM
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Default Re: Brake booster and PCV hose location? (fauxrs)

It is an open plenum! Hmmm......

... thanks.
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Old 04-20-2003, 10:26 PM
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Default Re: Brake booster and PCV hose location? (paul79)

Paul, I noticed that from your brake booster the vacuum line connects to a 1" device (check valve? filter?) that is standing straight up. This weekend I noticed that mine is at a 45degree angle and not making a real good connection at that point. Does that device need to sit straight up? Looks like it will rub the closed hood? Can I extend that short hose to lay that device sideways out of the way?
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Old 04-22-2003, 11:22 PM
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Default Re: Brake booster and PCV hose location? (Dk Met Grn 80 L82)

Paul, I noticed that from your brake booster the vacuum line connects to a 1" device (check valve? filter?) that is standing straight up. This weekend I noticed that mine is at a 45degree angle and not making a real good connection at that point. Does that device need to sit straight up? Looks like it will rub the closed hood? Can I extend that short hose to lay that device sideways out of the way?
It does not mater which angle this is mounted at. My truck has one that is in line, but the vette does not have one at all.
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Old 04-23-2003, 08:09 AM
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Default Re: Brake booster and PCV hose location? (BSeery)

I had the same problem with my Demon. I had them T'd together and was told that was a no-no. Edelbrock sells a low profile vac 90 degree elbow that can be installed in the hole in the top of the intake right behind the carb. I had to use a 1/4" spacer to get the carb clear of the vac fitting. I now have the brake booster in the back of the carb and the pcv going to my new vac fitting in the intake. part # on the fitting is 8096.
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Old 04-23-2003, 10:29 AM
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Default Re: Brake booster and PCV hose location? (Paul 75 L82)

Edelbrock sells a low profile vac 90 degree elbow that can be installed in the hole in the top of the intake right behind the carb.
But isn't that whole for the Vac accessories and (in the case of an automatic) the vac line down to the transmission?
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Old 04-23-2003, 10:38 AM
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Default Re: Brake booster and PCV hose location? (tzavaleta)

Sounds like demon owners need to switch to the hydro-boost system to eliminate the need for the extra vac port plus you can run a huge cam and not have to worry about losing brake pressure.
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Old 04-23-2003, 09:23 PM
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Default Re: Brake booster and PCV hose location? (tzavaleta)

I don't recommend drilling an intake,but can tell you I teed my power brakes and PCV together,and they work fine. I went from an Edelbrock 1904 carb(Q-Jet clone) which had tons of vacuum ports,to a Holley 3310. Missing all those ports I was forced to try this and that till it worked out. For what it worth I ended up capping the ported vacuum source for the distributer,and running unported for that and the climate control stuff,and then off the large rear port I tied the power brakes and PCV.

The vacuum works like a diode in electronics. The flow is one way. The brake booster has a check valve which lets the vacuum in,not out. So the brakes pull whatever they can when ever the manifold vacuum is high,and the check valve stops the reverse flow of vacuum. The PCV valve only opens when the vacuum is low,like acceleration,and closes when vacuum is high,like idle. So when you mash it,the PVC valve gets what it needs,when you hit the brakes the power brake booster gets what it wants.

Or maybe not,in any case it worked for me.
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Old 12-08-2017, 10:48 PM
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Originally Posted by standup View Post
No offense, but I believe this is incorrect thinking.

PCV is an emission control system and works by drawing blow by gases (contaminents) from the crankcase, through the PCV valve and into the combustion chamber to be reburnt.

The brake booster draws vacuum to be stored in a reservoir for proper operation of power brakes.

The two systems are independant and shouldn't be combined, for two reasons:

1. Contaminents may be drawn into the brake booster, thus affecting your brakes.
2. An added T-junction will increase the risk of vacuum leak, thus affecting your brakes.

Your car will still run without a properly functioning PCV system, but your engine will accumulate water, acids and unburned fuel, which means quicker contamination of your oil (and more frequent oil changes) and increased wear and damage on the engine.

The PCV system is also linked to the fuel vapour cannister, which removes fuel vapour pressure from your tank to be reburnt in the combustion chamber. Severing this link means you have to use a vented fuel cap to relieve pressure at the tank or use the cannister to vent fuel vapour: both of which have their own disadvantages.

You don't have the ports because Demon (and other aftermarket carbs) are designed for racing applications, where the PCV and Fuel Vapour systems are not wanted nor required.

The real question is: How big is the risk in modifying your factory system?

1. If you "T" the PCV and brake booster, your brakes may fail.
2. If you use breathers instead of a PCV system, you may have to clean your engine bay more often, you're polluting the atmosphere, you'll have to vent fuel vapour through the fuel cap or cannister, which could create a source of ignition.
3. If you spend $20-40 on a carb spacer with a PCV port, it may not fit under the '74 rear induction hood. (BTW--The rear induction hood on your '74 isn't that much smaller than an L-88).

The idea is to run the PCV system as close to factory as possible, not because of NCRS, but because you don't want to invent a new set of problems.

I hope this clarifies the matter....and good luck!
DrMichael says, with all due respect, the break booster does not "draw vacuum" from the engine. Engine vacuum pulls air out of the break booster. The direction of flow is from the break booster to the engine, not the other way around. Therefore the break booster should never draw contaminants from anything. No?
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Old 12-09-2017, 12:34 AM
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Normally, yes. But, if the brake booster vacuum is high (and it has a check valve to keep it from going low) and the engine has a partly open throttle when the brakes are applied, the differential pressure could draw some PCV gasses into the booster. Not what you want to happen.

The brake booster and the PCV dump line both go to the throttle plate (direct input to the intake manifold). But, the PCV line is expelling vapors (to be burned with the fuel charge) and the brake booster is just looking for vacuum assist. It is BEST to keep them separate, but both need to have good sized connections and hoses. One could always drill and tap another fitting directly into the intake manifold for a "clean" connection, but the throttle plate is better for both.

Last edited by 7T1vette; 12-09-2017 at 12:35 AM.
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Old 12-09-2017, 09:24 AM
  #17  
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Originally Posted by DrMichael View Post
DrMichael says, with all due respect, the break booster does not "draw vacuum" from the engine. Engine vacuum pulls air out of the break booster. The direction of flow is from the break booster to the engine, not the other way around. Therefore the break booster should never draw contaminants from anything. No?
Exactly. By design and physics it cannot happen, period. The brake booster vacuum hose is never subjected to a positive pressure. Why? It's always under vacuum when the engine is running. So it's always sucking on the brake booster. Also, there is a check valve on the booster that controls vacuum pressure to keep the booster under a vacuum. At best, if the check valve fails, the booster would lose vacuum and be at atmospheric pressure BUT only when the engine is NOT running. And that still won't contaminate the booster whatsoever.

Edit: This post is 14 yrs old, lol. Also, I don't know why anyone would even consider cross connecting the two systems.

Last edited by resdoggie; 12-09-2017 at 09:28 AM.
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