A/C Air will slowly start to blow via defrost but not what you think

[FHP89LX]

Hey guys, as usual, I checked around the forums here to see if my specific issue has been discussed here but not exactly my issue was found here so here it goes.

I have 2010 Mountaineer 4.0L. I turn on my AC and drive for about 15 minutes and the ac is blowing perfectly through the center and outer vents. After 15 minutes or so, the air coming out of the vents blow out less and less. The more I drive, the less comes out and then it just blows out through the defrost vents. The blower speed is the same.

If I turn off the car and come back 20min later and drive off with the AC on, it will work perfectly for about 10-15min and then start to blow air through the defrost vents. I checked behind the dash and the piece the open and closes the door is in perfect condition.
This also happens when I am driving on the high way cruising at 60 -70mph and it will start to blow via defrost vents but then after 5 to 10min it will blow out through the main vents.
It sounds to me like a vacuum issue but I heard that possibly it could be the check valve like this one: https://tinyurl.com/y8g2t3u9
I am planning to buy the check valve but would love to hear from anyone that has had this issue.

 

[BrooklynBay]

Check the hard plastic vacuum tubing, and the rubber boots for cracks.

 

[Larryjb]

There's a check vacuum valve near the firewall that goes too. That's what started leaking on mine.

 

[BrooklynBay]

You could find that part in the Dorman/Motormite/Help section in an auto parts store.