Brake Light works when it wants 2

On my 97 blazer, for some reason sometimes the left brake light will not light up when the break pedal is pushed. Its not the bulbs, done tried that. I did follow the wire 2 the front b4 it runs up under the dash and for some reason the wire still shows voltage when the pedal is pushed but wont light up the brake. HAs anyone else had this problem????

Have you verified voltage at the bulb?  The bulb sockets on these trucks can be known to corrode internally and not make proper electrical contact with the bulb.

i thought that that would be the problem, but i just shaved my tail lights and put in 2 15" led lights and its still doing it.