Chevrolet Repair: vaccum lines for switching vent to defrost to heat, chevy caprice classic, chevy caprice

Question
I have a 91 chevy caprice classic that blows air out of the front defroster the vents and the floor at the same time (not strong). I cant switch it to defrost wich is a pain in the winter. I put a new engine in the car (Brand New 5.7 ). Everything else is stock, including all hoses lines intake (TBI)everything. I took apart the dash and exposed the heater core. There are a lot of little vaccum lines and a switch that distributes the vaccum to the 2 different diaphrams that controol where the air blows out of. THere are two "main" vaccum lines that go through the firewall one goes to the "valve thing" that allows hot water to go through the heater core. The other goes who knows where? (possibly to the throttlebody) WHen i suck on the main vaccum lines i can get the diaphrams to switch between floor and defrost and vent. I dont have air conditioning since its not hooked up but living in Erie PA who cares. THe other vaccum line seems to hook to the front of the Throttle body(it might have a leak in it). Before i go tearing into the wireing harness looking for the leak in the line is there another line that i havent thought of or another problem that might exist or am i on the right track at all.
The heat works fine just conttrooling where it comes out of is the thing. THanks for yer help
Charles Utz  

Answer
Hello Charles, There is a vacuum line that connects to the intake manifold or the TBI unit that should have vacuum all the time. It goes to a check valve on the drivers side of the firewall. Find this check valve and make sure that there is vacuum to it and going throught it. This is where you should find your problem.

Hope this helps

Mark