Vacuum problem

[am3rican]

I have a 1968 plymouth roadrunner with 440 6-bbl. It had factory manual brakes. I recently converted to power disc brakes, all four corners. At the same time, I replaced the throttle body gaskets on 2 of the 3 carbs.

My booster requires 16-18 inches of vacuum to operate properly. With my stage 3 crower cam, I'm getting 12 psi at idle. I checked for vacuum leak and could not find one.

Couple questions:
Is it normal for vacuum to increase when in Park? It will drop from 15 to 12 inches when in drive.

I disconnected the hose to the power booster and plugged it, vacuum dropped to 5. Not sure if this is normal.

I want to avoid getting a vacuum pump. Will a canister get me the difference?

 

[Hemi Rebel]

If you have 15 inches of vacuum in park, then you disconnect and cap the booster hose, how does it drop to 5? It should still have 15.

 

[am3rican]

If you have 15 inches of vacuum in park, then you disconnect and cap the booster hose, how does it drop to 5? It should still have 15.

Not sure. I have a vacuum gauge. The moment I disconnected the vacuum hose and capped it, the vacuum dropped way down and almost stalled. I admit i'm not using vacuum hose (yea I know) but the hose isn't collapsing.

 

[coloradodave]

The vacuum probably dropped because the RPM came down. Same with Park verses in gear. BTW, my motor has 14-15" of vacuum at idle (4 speed) and the booster works just fine.

 

[Yatzee]

That's a bit low depending on the square area of the booster diaphram. On the smaller ones like 7" vacuum works like it's counterpart pressure. A reserve can may help but 15 is 15 no matter if there's a vacuum can or not. Could try a larger diameter booster but it's a crap shoot at best.

 

[lewtot184]

i went thru something similar to this recently. i could never figure out why my power brakes never worked right at low speeds. done some testing with a vacuum gauge. had 16" in neutral and 12" in gear. i ended re-curving the ignition, picked up some vacuum in gear and solved the problem, i think 12" is too low to make the boosters work right.

 

[Shorthorse]

Performance cams have a definite effect on vacuum. Factors like duration and overlap play a significant roll. It would be best to contact Crower and discuss your setup with them.

 

[Yatzee]

Just an additional note to my previous post. Having exactly the same issue as you did, and not wanting to keep my car exactly original, I opted to install hydro-boost running off my Saginaw power steering pump. The brakes are absolutely fantastic and feature backup safety features built right into the unit. I'll never go back to a vacuum system and certainly won't give second thought to the amount of vacuum I have at the manifold.

 

[dave5237]

Just an additional note to my previous post. Having exactly the same issue as you did, and not wanting to keep my car exactly original, I opted to install hydro-boost running off my Saginaw power steering pump. The brakes are absolutely fantastic and feature backup safety features built right into the unit. I'll never go back to a vacuum system and certainly won't give second thought to the amount of vacuum I have at the manifold.

Pardon my ignorance. What is hydro-boost. I am curious. Thanks.

 

[snakeoil24]

hydraulic brake pump

 

[Hemi Rebel]

Pardon my ignorance. What is hydro-boost. I am curious. Thanks.

Brake assist powered off the power steering pump. Most commonly found on diesels.

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Lets start over for a second so i understand correctly. The car ran fine with manual brakes right?
Try this. With it idling and the booster vacuum line connected, pinch the line off with a pair of pliers. See what the vacuum guage reads. See if it goes to 5 inches

 

[am3rican]

I called crower and they never called me back. I did speak to the vendor who sold me the kit. They mentioned a larger diameter booster will not help. Their tech recommended I get a vacuum pump. I was wrong. They said I need a minimum of 18 inches/hg of vacuum, not 16.

I'll try the test with the gauge tomorrow and will let you know. Car ran okay with manual brake. Keep in mind I replaced the carb gaskets when I did the brakes. They were brittle and leaking fuel and soaked with fuel. So it didn't run great. It had lower vacuum too, but I just assumed it had a vacuum leak because of the gaskets.

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Also, I want to install a pump. I just need something that is an easy install. Any recommendations?

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Anyone know of someone in Philadelphia PA area that knows how to tune a 6bbl?

 

[snakeoil24]

Check your center carb with carb cleaner. spray around the shaft and see if vacuum increases. I would use the cap vacuum pump it uses a really trick sensor.

 

[D575]

Went the vacuum pump route, mounted under the battery tray.

 

[mopar 3 B]

I am using a small factory booster at 10 hg. with no problems. You using some kind of aftermarket booster?

 

[am3rican]

Yea I bought the kit from Pirate Jack. Requires a min of 18inhg

 

[mopar 3 B]

Did you know that before the purchase? Can not imagine why companies continually sell stuff that will not fit the application and with all the stupid questions asked when buying parts these days.

 

[am3rican]

Do you guys have part numbers for the vacuum pumps?

 

[am3rican]

Did you know that before the purchase? Can not imagine why companies continually sell stuff that will not fit the application and with all the stupid questions asked when buying parts these days.

I did not know.

 

[am3rican]

Guys, Quick update- thanks to snakeoil, I ordered the vacuum pump, switch, and module. He was kind enough to provide part numbers. I need to buy a fuse, some wire, and I will tackle it. How did you wire it such that it only ran when the car was on?