On my 2003 Ranger 3.0L whenever I go up a hill, I loose all the air coming out of the vents. It will only come out of the defrost and I believe floor. Then when I get back on level ground, it starts blowing out the dash vents again. Any ideas what it could be. Im thinking a vacuum leak somewhere.

Thanks you guys:):)

 

I'd be looking for a vac leak as well.

 

Welcome! Yes it is caused by a vacuum leak. When mine did this it was because of the vacuum line that went into the heater valve had come loose. Check there and follow it for any breaks from one end to the other.

 

Is this a recent issue? Another possibility is that a previous owner removed/bypassed the vacuum reservoir.