99 chevrolet truck a/c problem

The belt on my A/C keeps breaking, ihave replaced the belt 4 times, also replaced tensioner pulley that the belt rides on. still same problem, My mechanic tells me that a shorter belt will do the trick so we try that and still the same thing belt breaks, Isuggested to the mechanic that maybe the compressor pully is intermittinly locking up and causing stress on belt, he says no,Can some one please tell me what will fix this problem before i spend a bundle on a new A/C compressor.