stupid air conditioner

so after rebuilding the entire front end of my celica.....I decided I don't need the air conditioner so I dumped it...well the last week has been in the 100's and I need it back...So I got the system back put it in....but when the ac compressor kicks on it only stays on for like 4 or 5 seconds, and then it seems like the high pressure switch is turning it back off, and then my ac light flashes on and off. for some reason.....now in the 5 seconds that the compressor stays on. you can feel the air beginning to get cold pretty quick. so my question is...what is causing the ac to shut down down?? Could it be that high pressure sensor?