Do you have the vacuum can out or is it still mounted on the car?  Rust eats at the seams of those things and causes tiny pin holes or cracks around the rolled seams that may not be visible with it mounted in the car.  If you have it out of the car you can plug two of the outlets and use a short piece of tubing to blow smoke into the can to locate the leaks.  If the holes are not too numerous or big, you may be able to seal them with fiberglass resin to see it that will stop the leaks.  The fiberglass resin will probably be a shot lived fix though, if the can is rusted in one place, there are probably many other places just waiting to fail.  While you are under the car, follow Micki's advise and  check the relays.   As the hoses get old and hard, they may not seal on the relays like new hoses.  Use the guides at corvette-101 and take it step by step to get them working.   Many of us here have already run that rabbit, and the rest will eventually. Crossfire19822008-03-24 20:34:55

A good way to find a leak is to let the car idle, then spray a little ether or carb cleaner on suspected leak areas.  If you hit a leak, the engine will speed up.  Work slowly along hoses, as it will take a moment for the ether to get to the engine as you get farther away.

Larry

not trying to be a smart butt here but are you sure the cannister is leaking? hook a vacuum guage to one side and a vacuum pump on the other and see if it holds vacuum. the lights on my 75 drove me crazy for a while. they would work perfectly if i hooked one up at a time but not if i hooked up both. i followed the complete guide at 101 and it turned out to be a bad relay. wish i had the guide before i started on mine as i wouldn't have went as crazy as i did. hope you get them fixed.

I believe the vac reservoir on the '75 is the whole front bumper support bar...not a "can", as the older, or even newer cars .
You'd be hard pressed to "see" a hole in that thing anyway!
Best suggestion is to hook up a vacuum pump to it, put a vac. gauge on one of the fittings, and block off the other fittings. See if the thing will hold a vac. If not, they can be welded, if you can find the leak. If it does hold a vac., then your leak is somewhere else. The most common area for a leak would be the relay valves, the actuators, or the one-way check valve at the engine vac. source.
hth

If your reservoir has tiny leaks in it you can probably just hit it with a couple of coats of POR 15 paint that stuff will seal up a wiffle ball

How i checked it was I plugged two of the vacuum ports on it and then hooked a vacuum gauge to other and pumped it I could not get any reading off of it at all...

Another option would be to block off all but one of the ports on the tank and apply air pressure instead of a vacuum.  If there are any leaks in the tank, they would be easier to find with the old soapy water test on the outside of the tank.  Just an idea. Dejablu2008-03-25 21:00:58

[QUOTE=Adams' Apple]I believe the vac reservoir on the '75 is the whole front bumper support bar...not a "can", as the older, or even newer cars .
You'd be hard pressed to "see" a hole in that thing anyway!
Best suggestion is to hook up a vacuum pump to it, put a vac. gauge on one of the fittings, and block off the other fittings. See if the thing will hold a vac. If not, they can be welded, if you can find the leak. If it does hold a vac., then your leak is somewhere else. The most common area for a leak would be the relay valves, the actuators, or the one-way check valve at the engine vac. source.
hth[/QUOTE]