On my 94 Z28 the heat and air condition works great but when I switch it to blow to the top vent not the defrost it takes for ever to come out those vents and on my friends 96 it does it instantly I think I might have a problem with the vacuum system any ideas.

 

You probably have a slow leak in one of the lines.

An easy way to tell what part of the system may have trouble is to go to the vacuum check valve and test the lines back to the mode selector and to the vacuum canister. If you have a hand held vacuum pump, it would be pretty easy. What you would want to do is remove the hoses one at a time and apply vacuum to each and see if it holds. The line to the intake manifold will have to be removed at the manifold and plugged back toward the check valve for you to test the line for leaks. You can test the check valve, too. Sometimes they are the cause of a leak.

If you find a leak on a particular hose, you will have to trace out the cause on that section.