That all sounds correct, HawkRod. If true, that would be an additional benefit of the vacuum reservoir on early C3 cars, and would be one more consideration for the OP, since he is asking about his 1968. As I said, I'm not that familiar with the vacuum wiper door system.

Nevertheless, as the vacuum wiper door was abandoned prior to the 1973 model, the only purpose of the reservoir thereafter was to ensure continuous vacuum in the event of wide-open throttle. So that benefit would be lost on all C3 cars without the reservoir.

I ran some more practical vacuum tests today on my '79. I observed that once I shut the engine down, I have about 20-30 seconds left of residual vacuum. Since wide-open throttle somewhat simulates engine shutdown, as far as the vacuum system is concerned, that is how long I have before I lose vacuum entirely. That is what keeps the headlamps from popping up when you floor it. Continuous vacuum keeps them down and in place.

 

are you saying that after shut down without using vacuum for any purposes the system loses any vacuum it has in 20 to 30 seconds?

 

No, I'm not saying that. Several times today when I shut off the engine, I would then immediatly activate the headlamp buckets as a test to see if they would raise and lock, knowing that vacuum was no longer being generated. The result each time was the headlamps would come up maybe half way, and slower, then stop, hold briefly, then slowly sink back down flat. The entire process took about 30 seconds from engine shut down. After that, they would not move at all unless I started the engine aain, when they would again function normally.

This leads me to believe that the reservoir is soley for the purpose of maintaining vacuum under hard acceleration, consistent with the '79 Corvette shop manual, for all '73-'82 cars. Prior to '73, I allow that there may also be enough vacuum briefly remaining after shut down to close the wiper door, as described above by HawkRod. If so, then the reservoir serves two purposes on '68-'72 cars.

 

Gotcha. My take. vacuum is created within the vacuum system by the engine. When engine vacuum drops to a certain point check valve closes and vacuum is kept in vacuum system....until it comes out...somewhere somehow...being used as designed or leakage. In "theory" it stays in there forever if system is perfectly tight. If check valve does not close then the intake etc remains part of the vacuum envelope and vac reduction from hard acceleration results in vacuum reduction in the entire vacuum system and does not allow the reservoir to add any value to the system. Lack of something called out in the shop manual does not mean it doesn't exist. I think you have a vac system leak...but maybe I am wrong.

 

My parents bought a new 72 Buick Riviera that had a vacuum operated switch to open the trunk lid.
One day when we were bored, my older brother and I talked our little brother into hiding in the truck and we promised to let him out.
It was fun for about three or four times until the switch ran out of vacuum and locked our little brother in the trunk.
It was a couple of hours before our parents got home to let him out.
We got in so much trouble from our mom, but for some reason we could hear mom and dad laughing afterwards in the other room.
Just a good memory to share.

 

Just a note, my new to me but used 2 port 68 vacuum tank should be arriving today via eBay. The one part I don't get is the hard acceleration and head lights popping up. As I put new vacuum lines in, there is a line to open the headlights and a line to close the headlights and my understanding is that one or the other activates based on the vacuum relay at the nose of the 68 and that relay is triggered by the light switch. My understanding of the headlights is that they will stay either open or closed regardless of vacuum. If I turn the car off open, they stay open and if I turn the car off closed, they stay closed and do not respond to vacuum unless I change the state of the light switch.

 

that is my understanding. They stay open or closed mechanically..not by continuous vacuum.

 

I really hesitate to comment further, because I frankly don't know, or care to know, how these vacuum systems work, and why I have eliminated all of them from my cars. But......I am pretty sure that the vacuum "SWITCHES" are like other switches....you have to turn them off or on, to change the current state. Once the headlights are up, or down, vacuum can be removed and the lights stay where they are......there is an obvious cam-over action of the mechanism that has to be overcome by a force, in this case, vacuum, to move them to the other position., either to open, or to close them, once they are in the opposition position. Icould be wrong. As for the crazy wiper door system on early C3.....I know nothing about it, and won't participate in it. My 69 will have the door, and it will work.....but its going to be purely mechanical.....I am not getting involved in any vacuum or electrical solutions.

 

seems to me you understand perfectly how they work.

 

Engine vacuum pulses with each stroke and varies with throttle opening. The reservoir tank and the one-way check valve are to provide GOOD vacuum during driving, regardless of the present state of the engine. Is it absolutely necessary? No. It is beneficial for consistent operation of vacuum systems? Yes.

 

That is correct if the engine is off. The headlamp doors will stay open or closed because there is no force acting on them. On the other hand, if the engine is running, it gets slightly more complicated.

The C3 headlamp doors are spring assisted and vacuum actuated. If working properly, the headlamp door assemblies will mechanically lock into place in the raised position, but not in the lowered position. When you raise your headlamps, you can hear a distinct click when they reach full upward travel. That locked position keeps them raised whether the engine is running or not. When the engine is running and you turn the headlamps off, vacuum force, as CorvettePassion contemplated, overcomes the lock mechanism, and the headlamp assemblies are then free to travel downward.

The headlamps do not lock when down but remain closed as long as no significant change in vacuum affects them, such as when the engine is off. However, with the engine running and, specifically, when wide-open throttle occurs, the significant and abrupt drop in vacuum could briefly affect them in the (unlocked) down position. This effect could cause them to lift somewhat. What is the solution to this issue? The addition of a reservoir tank ensures that, during hard acceleration, there is ample vacuum consistently available to systems needing it even though manifold vacuum becomes a roller coaster.

 

FYI.......I ran the 77 for years without ANY vacuum hooked up to the headlights, and they stayed down all the time.....did not pop open. And when I manually pushed them up, they stayed up until I pushed them back down. Never did they once move. This is how I will run it now,...and will run my 69. So.....tells me that vacuum has nothing to do with maintaining them up or down.....only used when you want them to move.

 

True, if you have none vacuum to the front of the car. The headlights will stay down.
but if you have vacuum connected to the actuators and the relay switches. That's a completely different scenario.
The vacuum relays that control the actuators are spring loaded. With vacuum applied. The vacuum through them is directed to the close the headlight side of the actuators. When there is no vacuum to the relay a spring inside pushes the dog bone to direct vacuum to the raise side of the actuators.
When you pull that over ride switch under the steering column or pull the headlight switch out. You shut off the vacuum to the relays that control actuators.
NOW, if you have none vacuum to the actuators as in disconnecting all vacuum to the front of the car. Yes, nothing will happen.
BUT! And this is a VERY common issue. If everything is connected and you have a bad check valve or leaky reservoir. Than that very small line that runs through the over ride switch, and then the headlight switch and then all the way to the front of the car will drop vacuum far quicker than the large line to the actuators. And when that small line drops vacuum the spring inside the relay switches pushes the dog bone down and directs vacuum to the raise side of the actuators.
And the headlights wink under acceleration.