Chevy & GMC Vans

Got a 2012 GMC Savanna 2500.Recently the ac started not blowing out the vents going up sloped roads or hills but once I am back on leveled road it starts coming back out the vents.You can hear the air inside the dash areas but just does not come out the vents.Sometimes just for the heck of it I will let off the gas pedal and the air comes out the vents but when I hit the pedal again it stops Today it has stopped completely coming out the vents and now it just blows up in the dash area,out the floor vent and the defrost vents.Any ideas?Thanks

replace your vacuum lines, the vacuum tank, and the checkvalve. That should bring everything back.