In my 88 S/C 305 when I turn the slider to the off position my heater still blows mostly out of the floor vents but a little out of the top vents. Should I start looking for q vacuum leak?

 

Thats a good place to start. However, when your climate control loses vacuum, it usually blows out of the defrost vents. Sounds more like you've got something wrong with the mechanism itself.

 

Is it coming out both lower vents, or mostly the passenger side?

 

It comes out both driver and passenger side.

 

TTT

 

Did you check to make sure everything's connected to the heater/AC faceplate as they should be? The H/AC system is kind of touchy... i'm still trying to fix mine after my heater core replacement. (Didn't line my panels up quite right, so everything blows out the passenger side floor.) If I remember right, there should be two multi-line vacuum ports, a mechanical metal line that connects to a switch arm, and an electrical port... if any of those aren't hooked up, or aren't hooked up and lined up right, it'll wreak havoc with your system. If the only problem is that it keeps blowing when it's off, I'm not sure what it could be... mine does that intermittently. If I play with it just right I can get it to shut off entirely, but it's not easy. :P