My A/C will only blow through the defrost and floor vents. This has been a progressive problem. When it started it blew through the defrost/floor vents only when I was driving on the highway and through the dash vents when I was stopped or going slow. Now it's through the defrost/floor vents all the time. I read some older posts about this problem and the consensus seemed to be that it was a vacuum leak somewhere. I've looked under the dash on the drivers side and behind the glove box on the passenger side. All the vacuum lines seem to be in proper working order (connected properly, not cracked or dry rotted, etc.). Although I could only trace 4 of the 6 lines. I then looked under the hood. Assuming I'm looking at the correct lines, I traced them from the intake manifold to the vacuum reservoir. All the boots appear to be seated correctly and not cracked. All the lines appear to be firmly in the boots and not pinched anywhere. I did notice, however, that the boot that connects the vacuum line to the manifold is real loose (where #2 connects to #5 in the below picture). It takes almost no effort to pull it off.
So the questions are: is that the problem? Could it be a check valve? If not, what else could it be or where else can I look? Is there a way, other than visual inspection, to see if there is a leak somewhere?
Thanks for your time.