Hello Corvette people.
My father has a 79 corvette, 454 with a vacuum problem. I think his vacuum tank(big tube under head lights) has a bad leak but I can't confirm that. The only way to get vacuum is by running the car which makes it too loud to hear any leaks.
Using a hand held mightyvac, i can not get a vacuum reading on it but that might be due to the volume. I can bring the lights up and down by connecting directly to the front or back actuator hose.
The vacuum at the hose going into the holding tank is 20Psi or I assume it is reading in Psi.. Would it be OK to just bypass that big cylinder and connect directly to the relays or is that too much vacuum and I might damage a diaphragm?

Thanks for your help and guidance.

 

vacuum is measured in inches not pounds.
the tank already has the maximum vacuum it acts as a storage system for times when vacuum drops.
watch this video on diagnosing issues with the headlight system.

 

You first need to determine how much vacuum your engine makes in order to know how much leakage you have. If you remove and plug ALL of the vacuum line coming from the carb and intake manifold (except the distributor advance can line), you can then fire the engine and measure the MAXIMUM vacuum capability of your engine. Compare that to the reading you got [at idle] with everything hooked up. If there is a substantial difference, you need to work thru the diagnostic approach listed above to find the leakage source.

It is also possible to have leakage at or around the carb, carb base and/or throttle plate shafts. When you check for MAX vacuum, use a spray can of carb cleaner to shoot little puffs at various areas on the carb. With engine running, it will rev up a bit, if the carb can draw the carb cleaner thru areas of leakage. It's an easy way to find leakage at/around the carb....or find that there is negligible leakage there.

 

Hi, you didn't say what makes you think that there is a bad vacuum leak, I'm assuming its to do with the headlights because you say they work okay using your tester. If you block the outlets from the vac tank you should be able to achieve a vacuum reading with the tester connected to the inlet. Another way is to block the outlets and then pressurise the tank with the tester,(I'm assuming your one does both pressure and vac) don't overdo it if you use a compressor the tanks are quite thin. If the leak is as big as you say then it should be obvious otherwise you can use a soapy water solution painted onto the tank to find the leak, there will be a bubbles at the leak point. FWIW the tank in my '70 split slightly along one of its seams, everything still worked but it made the engine idle uneven.

 

Have you replaced the hoses ?

 

Suggestion: Replace nothing until you know that it is actually defective.

 

If I disconnect before the filter device, the vacuum is 20 inches.
It does not loose any vacuum when I measure it at the hose connecting to the holding tank and it holds steady. The engine idle is solid and does not fluctuate at all. Puffs of starter fluid around the carb dont increase idle.

My diagnosis that the problem is the holding tank is leaking, is that there are three nipples on the tank, One that the vacuum hose from the engine connects to (drivers side of tank), and one located next to each of the relays. There is almost no vacuum on either of the two hoses connecting to the relays. That is why I think the tank cant hold a vacuum.
So i guess my question still stands, is it detrimental at all to just Tee off the vacuum hose coming from the engine and connect directly to the center nipples on the relays?
Second question. The video (excellent video and diagrams, I can not thank you guys enough for such excellent information) did not state what the vacuum reading should be at the top of the relay (coming from the cut off and on off switch) so I dont know if I have enough to change the relay position. I did clean the cut off switch and it operates smoothly now but cleaning it had no effect on anything.
Third question : How much vacuum should I have coming off of the holding tank? It should be the same amount as I get coming off the engine right (20 inches)? or is there some sort of loss due to the volume of the tank? I dont know.. it should not but.. I have to ask.

I will paint it with soap and water but i'm not too sure how I will see a leak if the tank is sucking air, not blowing air (like a leak in a tire).
Those hoses were all replaced at some point, there is no dry rot anywhere and they are nice and pliable.

Thanks again for all the suggestions and guidance. I'm really glad to have found this forum.

 

To test the holding power of the vacuum reservoir tank, Connect the vacuum IN line to it; block off one of the other nipples and connect a vacuum gauge to the other one. Fire the engine and let vacuum stabilize. Compare that vacuum level with what you got with your gauge connected to ONLY the vacuum IN hose. If close to the same, then shut down the engine and watch how quickly vacuum is lost. Do the same thing again, but this time use a heavy duty squeeze clamp to close off the IN line BEFORE you shut the engine off. If it drops off at the same rate, you tank is leaky. If not, you have a bad one-way check valve in the vacuum IN line.

 

Plug off two of the nipples , and positive pressurize the tank, then soapy water will show you where the leaks are. Kinda like finding a leak in an inner tube

 

Ahh, Its like I look for a reason not to understand something simple... of course, pressurize it. Thank you for not laughing at me, I get tunnel vision some times.

I have not checked the check valve, simple enough.
thanks again for the help. It will be a while before I can touch it again but I will update when I do..

Side note : : " HI DAD!!"

 

Don;t put more than 5 psi in that tank or you will split it. It's designed for vacuum, not pressure. If you have a hand vacuum pump (Miti-Vac), its easier to test holding vacuum. But, either way will work....holding pressure or holding vacuum. If the tank is installed in the car, it would be impossible to look for soap bubbles.

 

Good to know about the 5PSI.. I would hate to have put the compressor hose up to it.
That is what prompted my inquirty about bypassing that tank entirely.

Can I safely bypass that tank and run the vacuum directly to the relays or will that damage the relays?

 

You really don't understand the vacuum system. The reservoir tank is there to maintain vacuum for headlamps, wipers, HVAC system, etc.during WOT operation. Under full throttle, you have NO vacuum....unless you have a vacuum reservoir with a check valve in the line.

You can do without the reservoir if you don't mind all the vacuum-operated stuff to go dead for a few seconds.