Vacuum Tank Question
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Joined: Oct 2010
Posts: 54
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From: Pineville, PA
Vacuum Tank Question
I have a 70 Vette with a show vacuum leak. If a C3 sits for a week, should it still be holding vacuum in the tank or does it bleed out completely over time? Thanks for any information.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If your tank holds vacuum for more than 30 minutes, you have a special car. 
The tank is [usually] not the problem; there is some leakage via seals in all of the vacuum relays and switches in the system. When new, it would hold vacuum for several hours. Forty-plus years later.....ummm....not so much. When the engine is operating, if all the vacuum systems work reasonably well, you are in good shape.
The tank is [usually] not the problem; there is some leakage via seals in all of the vacuum relays and switches in the system. When new, it would hold vacuum for several hours. Forty-plus years later.....ummm....not so much. When the engine is operating, if all the vacuum systems work reasonably well, you are in good shape.
Is my vette stock?? HAHA
Joined: Oct 2015
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I have new actuator relays and they still leak down after a while. Nothing built today is a good as the original vacuum system parts. I cut the ends off my old vacuum hoses to make sure I got a good seal and replaced the hoses that where dry rotted. It will hold for over an hour with the actuator lines disconnected and plugged, but when i hook up the actuator relays it gets a slow leak. But they still last over a 1/2 hour. If all they systems work and you have extra length in you lines, trim off 1/2" to 3/4" of the lines that are loose. It will help.
If you have a bigger cam you can add a hella rotary vacuum pump, they are pretty quiet with the hood shut and the engine running, and it helps keep the system at 15"hg. It has a vacuum switch that kicks on about 12" and off at 15" my car only had about 6 to 8 inches at idle so it helps a lot. 15" will make the headlights snap open. It comes with one way check valves to help prevent bleed back
If you have a bigger cam you can add a hella rotary vacuum pump, they are pretty quiet with the hood shut and the engine running, and it helps keep the system at 15"hg. It has a vacuum switch that kicks on about 12" and off at 15" my car only had about 6 to 8 inches at idle so it helps a lot. 15" will make the headlights snap open. It comes with one way check valves to help prevent bleed back
Last edited by Rescue Rogers; May 23, 2020 at 07:28 AM.
Le Mans Master
Joined: Jan 2007
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From: Omaha NE
2025 c3 ('74-'82) of the Year Finalist - Unmodified
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I bought my Vette new.
Even back then I remember most of the vacuum gone by the next day.
Even back then I remember most of the vacuum gone by the next day.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,150
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From: US-PNW
There are just waaaay too many slip-fit connections to even expect long-lasting vacuum for any appreciable length of time. Then throw in 40-50 years of aging of rubber diaphragms and hoses! As long as the system works when the engine is running I'm happy.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,239
Likes: 9,381
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
my headlight actuator relay finally quit the other day, oct 67 build, original relay. The wiper actuator relay I replaced 5 years ago crapped out at the same time.....no quality improvements in 52 years, it actually got worse
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The aftermarket only attempts to make their parts LOOK like the originals. Very few of them are built of the same quality materials, machined to the same specifications, operate as well as stock parts, and they certainly can't get close to the reliability of original parts.
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Joined: Oct 2010
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From: Moscow Mills Missouri
I have new actuator relays and they still leak down after a while. Nothing built today is a good as the original vacuum system parts. I cut the ends off my old vacuum hoses to make sure I got a good seal and replaced the hoses that where dry rotted. It will hold for over an hour with the actuator lines disconnected and plugged, but when i hook up the actuator relays it gets a slow leak. But they still last over a 1/2 hour. If all they systems work and you have extra length in you lines, trim off 1/2" to 3/4" of the lines that are loose. It will help.
If you have a bigger cam you can add a hella rotary vacuum pump, they are pretty quiet with the hood shut and the engine running, and it helps keep the system at 15"hg. It has a vacuum switch that kicks on about 12" and off at 15" my car only had about 6 to 8 inches at idle so it helps a lot. 15" will make the headlights snap open. It comes with one way check valves to help prevent bleed back
If you have a bigger cam you can add a hella rotary vacuum pump, they are pretty quiet with the hood shut and the engine running, and it helps keep the system at 15"hg. It has a vacuum switch that kicks on about 12" and off at 15" my car only had about 6 to 8 inches at idle so it helps a lot. 15" will make the headlights snap open. It comes with one way check valves to help prevent bleed back