2000 F150 still having a/c problem

I have checked the blower, the resistor, changed the resistor and the a/c still won't come on. The fuse under the hood was good. The fuse under the dash is still good. What else could be wrong? Is there ain-line fuse that could have gone out? My brother had that happen on Nissan that he used to own. Any help is appreciated. Thanks:banghead:

Try jumping the low pressure switch found on or near the filter/dryer.