Possible Vacuum Problem?

I have an '87 GM that has the following problem. When accelerating, the fan blower slows to almost zero. On deaccel it picks up again. Perhaps more telling is that a great deal of hot air comes out of the defrosters when the ATC is in the "OFF" position! It's like none of the positions of the controls work correctly. Where is the main Vacuum Tank and Check Valve? Is it likely to be an individual vacuum line or a control unit? There is an 8 Port Vacuum Selector along with the Control Assy. Whick is the most likely suspect?? Thanks....MB

not sure where your tank is, but there is a single vaccum line going to your heat/ac controls. the check valve is in that line usally under the hood. should be able to find check valve if you trace that line. when you find that, dissconnect and check vaccum coming from it and see if it has a strong vaccum to see if tank is leaking.

Thanks for the "heads up"! Actually, the problem came from a couple of vacuum lines that connect from the Vacuum Control (attached to the ATC Unit). Once I replaced those the problem (at least at this point) SEEMS to be fixed!

That is kinda good to know since I have a simular problem, and I know it's a vacume leak, I'll have to take a look at my controls and see if the vacume lines are the culprit there as well.

Oh by the way the main vacume can is on the left fender well close to the brake booster if it is like mine it doesn't look like it would hold much vacume in it for long (Small can)... GRIN

Thanks for the "heads up"! Actually, the problem came from a couple of vacuum lines that connect from the Vacuum Control (attached to the ATC Unit). Once I replaced those the problem (at least at this point) SEEMS to be fixed!