Vacuum Leaks - Likely Sources?

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Apr 12, 2006 | 11:28 PM

icanfixitforyou

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Burning Brakes

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From: Same as our leaders Out of my mind

Vacuum Leaks - Likely Sources?

What are some of the most likely sources for a vacuum leak in a 79 Vette?
Thanks

Apr 12, 2006 | 11:38 PM

SIXFOOTER

Le Mans Master

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From: Boca Raton Florida

check all the vac lines attached to the carb and manifold AT the carb and manifold. The heat from the engine makes them get hard and crack with age. If it has been some time since the hoses were changed, it would be a good idea to run each one down end to end and verify that they are intact, mark them with tape as you go.

Of course check the carb to manifold seal and along the intake to head seals.

Apr 13, 2006 | 08:15 AM

Easy Mike

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From: Southbound

Cruise-In II Veteran

Quote:

Originally Posted by icanfixitforyou

What are some of the most likely sources for a vacuum leak in a 79 Vette?
Thanks

#1 -- Headlight operating vacuum system -- lots of places here for a vacuum leak.

Apr 13, 2006 | 08:28 AM

evalu8r

Racer

Joined: May 2005

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From: Amelia Ohio

Mine is a 79 and when I took the motor out in November going thru the "while I'm at it" illness, pretty much what wasn't a vacuum leak. Everything in the headlight circuit except the hoses. You can unplug the main hose at the T behind the carb and cap it off to test. Also check the heat riser valve passenger side exhaust and the heat valve just next to that. Then there is the vapor recovery can in the drivers side, I believe it T's off the PCV line. Then there is the heat riser thingy on the breather. Then the last one I can think of is the cruise control circuit. Those are the typical culprits.

Happy Hunting

Chris

Apr 13, 2006 | 09:28 AM

Wuttin

Drifting

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From: Hamilton NJ

On my 79 the 3 port tree for the vacuum fittings on top of the manifold behind the carb had been bubbafied when the intake manifold was swapped out by the PO. The line that inserts into the top of the tree comes from the vacuum modulator on the auto trans (assuming you have auto, steel line on passenger side). It was damaged during the refit since it's a pretty tight squeeze. Also, the small piece of vacuum line that connects the steel line to the modulator at the trans was spent on mine, making for a nice vacuum leak and a poorly shifting transmission.

FYI my pop-up headlights have been replaced with fixed boxes with all vac lines capped off...made the diagnosis a tad simpler

Apr 13, 2006 | 05:14 PM

icanfixitforyou

Thread Starter

Burning Brakes

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From: Same as our leaders Out of my mind

Thanks everybody, marking the hoses with colored tape is a good idea (and I have some in the basement). About the headlight vacuum circuit - Is it truly isolated from the vacuum supply by the headlight switch. Is there a vacuum line on our headlight switches? I don't even want to think about what I would have to do to take a peek at the back of the switch. What I am trying to say is - If the headlights are off - would a leak in the headlight circuit still affect total vacuum?

Apr 14, 2006 | 09:22 AM

ESU

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From: staten island ny

Quote:

Originally Posted by icanfixitforyou

I believe there is constant vac going to the headlight switch so when you turn the lights on the vac flows thru the switch to the headlights. A leak before the switch could be a problem. I had a leak after the switch with a line that was a little too flared on the end and every time I turned on the lights I could hear hissing. My suggestion is if nothing has ever been replaced get a catalog from Doc rebuild and start purchasing some of his kits. They all come with diagrams and color coded lines that are easy to re-install. You may find after 20 yrs of bubbaing like I did that many of theses lines are not hooked where they are supposed to be or just plain disconnected or capped. It took me about 6 months and 2 of Docs kits to get my headlights working properly and even got my cruise working.
Just my .02, ESU

Apr 14, 2006 | 09:25 AM

ESU

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From: staten island ny

Quote:

Originally Posted by icanfixitforyou

Oh Yeah, and after you get it all replaced or even before you start isolate all vac at the tree behind the carb. I mean cap them all including the brake line for the power booster if so equipped and take a reading with a good guage. Should be 15 or better depending on your engine/cam. Start hooking up systems one at a time and watch the needle on the guage, if it drops on hook up thats probably a system with a leak and one that needs your attention.
ESU