69 Corvette Vacuum issues
Thread Starter
3rd Gear
Joined: Mar 2021
Posts: 3
Likes: 0
69 Corvette Vacuum issues
This is my first post so let's see how it goes! Thanks in advance for anyone's help. I have a 69 vette, 350/350 with non-powered brakes and manual trans. I am having trouble getting my headlamps and wiper door to activate. My vacuum level off the manifold is about 14 and steady. I have replaced the wiper door actuator (it wouldn't work with a MityVac), the control valves for the headlamps and the wiper door, the check valve and the wipers down vacuum switch with no luck. I know my vacuum reservoir is not holding vacuum. The actuators on the head lamps do hold vacuum so I believe they are good. They have both been slow opening for quite some time now they don't open or close at all. I know that getting the reservoir out is a pain. Can I operate the system by bypassing the tank and hooking the two lines that feed the tank together? I have never changed the vacuum lines in all the years I have owned the car (since 77). Some of the connections feel very loose. I am kind of at a loss on how to efficiently troubleshoot the problem. Can it operate properly on 14 inches hg? Could changing all the hoses be the answer?
Melting Slicks
Joined: Apr 2010
Posts: 2,795
Likes: 1,196
From: Canberra Australia
2025 C3 of the Year Finalist - Modified
If you have tested the tank and it has a leak then just bypass it.
The system will still work but won’t have the tanks capacity to work from.
it is also easier to spit the system and isolate the headlights from the wiper systems while testing then just work through each component to see what does or does not work.
I purchased a small fridge evacuation pump years ago that makes the process a bit easier than using the hand pump.
The system will still work but won’t have the tanks capacity to work from.
it is also easier to spit the system and isolate the headlights from the wiper systems while testing then just work through each component to see what does or does not work.
I purchased a small fridge evacuation pump years ago that makes the process a bit easier than using the hand pump.
Last edited by CraigH; Mar 7, 2021 at 04:14 AM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
It is very common for the headlamp relays to leak internally which will cause slow headlamp bucket apply action. You can pull the 'yellow' striped vacuum hose off the middle nipple on a relay and connect it directly to one connection on the actuator servo. Depending on which fitting you connect to, the bucket will either raise or lower...without the relay being in the system.
If bucket action is still sluggish, the relay is NOT the problem. But, it is likely to actuate much more quickly from my experiences.
P.S. have you removed hoses and plugged fittings on the reservoir so you could pull some vacuum on the tank and see if it leaked-down or held vacuum? Easy to do with the Miti-Vac. If leakage is not too bad and you can determine the source of the tank leak, you can seal it up from the outside (if that source is accessible) since it holds vacuum.
If bucket action is still sluggish, the relay is NOT the problem. But, it is likely to actuate much more quickly from my experiences.
P.S. have you removed hoses and plugged fittings on the reservoir so you could pull some vacuum on the tank and see if it leaked-down or held vacuum? Easy to do with the Miti-Vac. If leakage is not too bad and you can determine the source of the tank leak, you can seal it up from the outside (if that source is accessible) since it holds vacuum.
Last edited by 7T1vette; Mar 7, 2021 at 09:01 AM.
Safety Car
Joined: Sep 2005
Posts: 4,226
Likes: 643
From: Orange County Ca
FYI It is VERY common for new replacement actuators, valves, etc. to be defective and leak profusely.
Check everything, don't assume that just because it's new, bought from a reputable dealer, that it's not chinese crap.
Check everything, don't assume that just because it's new, bought from a reputable dealer, that it's not chinese crap.
Thread Starter
3rd Gear
Joined: Mar 2021
Posts: 3
Likes: 0
Thread Starter
3rd Gear
Joined: Mar 2021
Posts: 3
Likes: 0
When I tested the tank with a MityVac it would not pull down, I didn't know if the MityVac has the capacity pull it down due to the volume. I just have to get a reducing barb fitting to connect the hoses to bypass the tank. I did buy a small vacuum pump but now need to get the fittings to make a hose to connect to the system.
Melting Slicks
Joined: Oct 2012
Posts: 2,156
Likes: 273
From: Dunedin NZ.
You should be able to pull the vac tank down with a hand pump. FWIW the vac tank is about a thin as a baked bean can so no surprises if its leaking. You have non power brakes? I believe you may be able to slip the tank out without disturbing the brakes, with power brakes the booster gets in the way. Someone who has done this could confirm though!
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Verdad Gallardo
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The Miti-Vac might pull enough vacuum to test the tank...but your hand may wear out first! Why not just start the car with the Miti-Vac (or a vacuum gauge) already plumbed, then clamp off the inlet hose (and shut the engine off). 5-10 seconds of engine running will suck it down plenty. Then watch from leak-down. You might also have some pieces of duct tape already cut to place over areas where you think leakage might be occurring. If the leak stops (or slows), that area likely needs 'help'.
