I have an 04 150 crew cab with all the bells...When I put my a/c on max the a/c blows for about 20 seconds then goes to defrost.It does the same when I put it on normal. However, if I put it on panel and floor mode it blows ok but only through the panel and floor.
Thanks for the help.

 

does it stop blowing cold air all togetheror does it just change from blowing cold air out the front and starts blowing equally cold air out of the defrost vents? if the air is staying cold but just changeing direction of the air flow i would say you got a vacuum problem. your problem sounds like a simple vacuum leak perhaps behind the dash on or possibly near the the heat / ac control panel check the vacuum lines at the back of the control panel and trace the back as far as possible to check for breaks or disconnected vacuum lines. There is a guy on here that goes by the screen name HAYAPOWER he can probably help you out as far as getting vacuum hose diagrams to see exactly what vacuum hoses go where and what they actually control as far as positions of air flow. hope you get it figured out.