Acceptable amount of vacuum leakage?

[Scott M.]

Hey guys,
in your opinions what would be the acceptable amount of vacuum leakage for a 72? I have been using a vacuum pump and when I remove it from the check valve, I seem to be losing about 1 inch every 4-5 minutes. I have been working on the headlight doors and wiper door system and replacing what needs to be replaced as I find it.
Thanks
Scott

[Rich W.]

Re: Acceptable amount of vacuum leakage?

Hey Scott ..I have had some luck with a vacuum pump..but not much..I have bought one from about every truck out there and they all leaked..some..plus the volume of air you need to suck down with one of those makes them kinda difficult to use..IMHO..I am a remove and plug type a guy..run the car and see how that works..I have found all the leaks on mine by doing it that way..as far as leakage..I would think none..it would be a great world if it really was ha!! I am not sure about leak down on these..but I am also not sure which exact part of the system you are having trouble with..I had problems with the wiper door..it effected the headlights..try that remove and plug method..let the car tell you where its leaking..

[Scott M.]

Re: Acceptable amount of vacuum leakage?

Rich, I am using a vacuum pump that we use to test refrigeration systems. It pulls down to about 28 inches and will hold the vacuum for longer than I care to find out. So it works really well and its very quiet! I have been removing the actuators and valves and pluging the lines, testing from the vacuum tank out. The tank is solid, no leaking there. I install a second vacuum gauge at the other end of the line from the pump and when I reach about 25 inches I pull the vacuum off the manifold side of the check valve. With all the lines attached to the tank and plugged on the other ends, the vacuum holds with no drop. Its not until I start adding the actuator relays (new) that I start noticing a slight drop in the vacuum. About an inch every 4-5 minutes. Just wondering if there was any amount of leakage that GM would have considered acceptable. You are right, in a perfect world there would be none.
Scott

[Patrick H.]

Re: Acceptable amount of vacuum leakage?

Many older, untouched cars will keep enough vacuum for hours or a day to open the headlights, if that gives you an idea. My 72 is able to do this.

Some of the repro actuators and valves leak; I've experienced this first hand. Test them all, don't trust them. Also, test the check valve.

Patrick

[Rich W.]

Re: Acceptable amount of vacuum leakage?

You're alot further ahead than I realized..I would do it the way you are if I had one of those vacuum pumps..I wanted to take the old AC machine apart at work but they wouldn't let me!! Patrick is spot on with those valves..I think they figure we are used to vacuum leaks so we don't care if these new valves leak a little..I haven't checked how long mine will hold a vacuum..but I can once I get the distributor done monday..I got a new vacuum gauge a few weeks ago I can hook up..anyhow..if you suspect a leaky valve it may be something to test separately ..

[Alan S.]

Re: Acceptable amount of vacuum leakage?

Hi Scott,
If you're still using the original hoses I found an improvement in the seal by cutting about a 1/4" to 1/3" off the end of each hose. There's usually enough length to do this, and you get to a 'fresher' part of the hose.
Then you only need to worry about all the seals in the relays and actuators?!?
Regards,
Alan

PS: When new I don't ever remember any winking or dancing going on; but then it was rare that the car sat more than overnight.

[Patrick H.]

Re: Acceptable amount of vacuum leakage?

Hi Scott,
If you're still using the original hoses I found an improvement in the seal by cutting about a 1/4" to 1/3" off the end of each hose. There's usually enough length to do this, and you get to a 'fresher' part of the hose.
Then you only need to worry about all the seals in the relays and actuators?!?
Regards,
Alan

PS: When new I don't ever remember any winking or dancing going on; but then it was rare that the car sat more than overnight.

You can use a tiny bit of black weatherstrip cement too, but ONLY do this if you're 100% sure that the actuator or relay you are attaching the hose to is fully functional and not to blame for the leak.

Patrick

[Scott M.]

Re: Acceptable amount of vacuum leakage?

Thanks guys!
Patrick, I checked out some photos of your car. Very nice! I think I may look through them from time to time to help get this thing back together.

Do any of you guys have a pdf copy of pages M215 and M216 of the 72 Corvette Assembly Manual? I was tabbing mine last night and noticed that particular page has escaped my binder.
Thanks again.

[John H.]

Re: Acceptable amount of vacuum leakage?

Do any of you guys have a pdf copy of pages M215 and M216 of the 72 Corvette Assembly Manual?

Scott -

Those "page numbers" weren't on the original sheets - each supplier of those reproduction manuals used their own "page numbers", so your numbers may not match someone else's '72 manual numbers for the same sheets.

If you'll use the UPC number and sheet number shown in the title block on each sheet, those will always match anyone else's '72 manual, regardless of how they were "numbered" by the supplier/reproducer.

[William C.]

Re: Acceptable amount of vacuum leakage?

M215-216 attached

[Scott M.]

Re: Acceptable amount of vacuum leakage?

Thank you Bill!

[Terry M.]

Re: Acceptable amount of vacuum leakage?

Hey guys,
in your opinions what would be the acceptable amount of vacuum leakage for a 72? I have been using a vacuum pump and when I remove it from the check valve, I seem to be losing about 1 inch every 4-5 minutes. I have been working on the headlight doors and wiper door system and replacing what needs to be replaced as I find it.
Thanks
Scott

Disconnect the vacuum going into the interior of the car for the HVAC system and then test. I think you will find a noticeable improvement. The HVAC control is simply two plastic disks rotating against each other with holes for vacuum passage. Those disks become lose and leak a bunch.

[William C.]

Re: Acceptable amount of vacuum leakage?

BTW, when I had my '72 (3* bowtie) about 15 years ago, I had it judged after a long drive. The car had been sitting for a couple of hours and the judges wanted to have the headlamps up. Without a thought i pulled the emergency up button under the dash and they popped right up. The judges checked the headlamps and wanted them down, I hit the button and they went down. Never started the engine.