2000 neon a/c

the a/c has a demon, I took it to the shop to have them check it (thinking it's needs some freon??) They tell me the a/c compressor is locked up?? OK! so i was driving the other day and I turned on the switch? AND to my suprise it's blowing cold air when pressing the brake pedal,it will stay cool for a few minutes then it warms up,touch the brake pedal, cool air agian??? I just don't understand how this could be happining? don't know whitch way to go..

Maybe a wire under the dash?That scenario is totally new to me.