A/C blowing only through the defrost vents

I have checked the actuator for the blend door and the door itself because I thought that was the problem, but they seem to be working properly. The one thing I have in mind is the vacuum is the problem but maybe someone could point me to a correct solution. While driving downhill sometimes, the max A/C will switch from just the defrost vents back to regular panels. I thought this was a fault in the blend door, so I had to confirm, however the blend door does open/close when the temperature is adjusted. I went to the one-way valve that is under the hood next to the battery and there only seems to be a problem on the intake side where the vacuum is pulling. I do want to point out that panel and floor used to blow cold through the regular vents and floor, but just recently all modes only go through the defrost/top vents. This is why I was leaning towards a vacuum problem. Any help with this problem that I might not be thinking of? I ran through the vacuum line that leads to the intake and it appears to be leaking somewhere. I was thinking about the tiny rubber elbow connector behind the manifold next to the firewall. Maybe someone else could steer me to an appropriate fix. I can also add images of anything if necessary. Thanks