1970 Corvette - when these cars were new would the headlight and wiper system hold vacuum indefinitely? What is a reasonable amount of time for a 45 year old car to maintain vacuum after the engine has been shut down? When the vacuum system has been purge (vented?) is it common for the headlight and wiper doors to open until vacuum has built up the next time you start the car?

Here's the deal.... I've been working all summer on finding and fixing my vacuum leaks. My system will currently hold vacuum for several hours. I still have a small leak somewhere on the command (small hose) side of the system. The leak is so small it's making it a little difficult to track down. The car will often sit for a week or more between starting the engine. After sitting for that long the vacuum system has lost pressure so when I start the car the LH headlight pops open along with the wiper door for a couple seconds until vacuum pressure has been reestablished, which drives me nuts. I was at a recent car show and asked these questions to other C3 owners and they all said they would be thrilled if their systems worked as well as mine.

 

New C3's would easily hold vacuum for several months but over time the spool seals in the relay valves and wiper park valve begin leaking and then the vacuum can be lost within a 24 hour period. Of the valves I would rate the wiper park valve as the most common problem.

 

Vacuum is the muscle that moves the headlight & wiper door. Vacuum does not hold them open or close. Once a door is in either the open or closed position (over center on the hinge) That door will stay in that position even when vacuum is lost or disconnected.

When you park the car in the garage with the headlight & wiper door in the closed postion, vacuum will eventually leak away (the vacuum tank actually fills with atmospheric air). When you start the car it starts to create vacuum, if the headlight & wiper door pop up for a second it means that the control valve to that system is compromised and is allowing muscle vacuum to the wrong side of the valve.

 

I understand how the vacuum system works, I've been working on it for two months. I'm just wanting to know how well it should still work and if my expectations that the 45 year old system should hold vacuum for several weeks are realistic.

I also want to know if after the system has been compromised if a door opening on start-up is common until vacuum has been reestablished.

 

this happened on my 69 so I replaced the check valve and that fixed it.

 

On my 68 there is only one headlight relay actuator in the front between the headlights, I replaced this and the doors work fine. They are not suppose to open upon starting. I go weeks at a time and longer and when I start the car the doors don't open unless I pull the light switch on. I might note that I had to replace this actuator relay twice because of the crap that is out there made overseas. I had to interchange the relay for the wiper doors to figure out that I had a NEW Bad Relay.

 

In today's world, this is very common. Only because of worn out vacuum components and most owners not understanding how to make the repair. The bottom line is the control side of the system leaks down before the actuating side does. When this happens with the engine running (continuing to produce vacuum), the leak is not apparent because the vacuum is continually replaced. It is primarily on shutdown that the imbalance occurs because of the lack of vacuum replenishment.

Another possible cause is if there is any vacuum loss around any of the rubber pistons inside the relays, thus not being able to direct actuating vacuum to the proper port. This is VERY common.

In theory, the system could hold vacuum till the cows came home. Many people have said this would happen to their cars when new. With the passage of time, and lack of understanding, the vacuum system has acquired a bum rap. If this is such a bad method, why do almost all new cars use vacuum systems in their hvac and transfer case controls?

Here's another problem. VERY BAD REPLACEMENT PARTS. Chinese relays and actuators are notorious for junk parts. Find or keep your original parts, and have them rebuilt. Outside of rear seals on actuators, these 2 components can be rebuilt easily. Pm me for details.

Willcox stocks a NEW domestic Made in the USA wiper door safety valve that actually works!

 

Back in the day my three year old 68 BB's vacuum systems worked flawlessly. I believe the vacuum systems are WAY past there design life and based on forum activity on them, somewhat troublesome.

 

Don't get me wrong, but: Is this an established fact? Are there any specs on this?

Having worked on UHV (Ultra High Vacuum) systems in a previous life, and considering the complexity of the Corvette vacuum system paired with the usage of many not-so-leak-proof parts (even when new), this sounds incredibly long... 'An hour or so' would have been my guess...

 

Thanks for the replies, I understand exactly how the vacuum system works, that is not the issue. I wanted to know what my realistic expectations should be. And I wanted to know when the system has been compromised if doors opening on start up is common as this seems a little backwards to me. It seems the vacuum should be supplied to the command side (small hoses) first, which would keep the doors closed.

The control side (large hoses) of my vacuum system is sealed and will hold vacuum for several days or longer. All of my valves, check valves, and actuators are new and have been tested. For the record two relays were bad when they arrived. One had a broken center vacuum port, the other was internal and the red port would not open.

The basic problem is I have a small leak on the command side (small hoses). The leak is so small it takes an hour for the leak to register on a vacuum gauge, which makes troubleshooting a long and time consuming affair.

And I know my leak is not in the wiper safety (park) valve as I isolated that section of the system and it holds vacuum. I've started isolating sections of my system and working backwards, sooner or later I'll find the section causing the issue, but working under a C3 dash is not the most comfortable place to hang out. And again I was just looking for answers to the two questions asked in the first paragraph.

 

The cheap safety valves now being sold by most vendors do hold vacuum, and sometimes very well, when the wiper door is closed or down. It is the open or raised position that these valves fail.

Have you checked the headlight vacuum valve? This is probably the poorest design in the entire vacuum system. You do not have to take the dash apart to check it. By being creative, it's simple to check.
Block the hose leading to the right side of the wiper door solenoid, pinch closed the control hose leading to the relays, and connect the vacuum pump to the small hose at the check valve. You can now pump up just the headlight vacuum valve and test for leak down.