Why would my air conditioner stop blowing air out of the vents. on 2003 Ford F-150

The only cold air that continuously comes out is through the defrost. It sometimes blows out of the vents when i let off of the gas and just coast. When i begin pressing the gas again, the air stops blowing.

Asked by for the 2003 Ford F-150
Have a mechanic find the vacuum leak from engine to HVAC system. Look under hood at the vacuum hose diagram, you may find it!
1 more answer
just repaired one that vacuum connector above the accel pedal come loose. these were also bad to break the hard plastic lines that run beside the battery if acid from battery gets on them.