why don't my brake lights work on my 1973 oldsmobile cutlass?

On every vehicle that i have had brake light trouble with, it has been a bad ground. Your taillights have a ground wire that goes to a bolt on your frame. The bolt rusts and then you lose connection. Chase wires til you find the ground, it shouldn't be far from the lights themselves.

an easy check is to use an ohmmeter at the light socket and check for an open in the circuit. if all four have an open, its more than likely a ground. if it was a short, your fuse would be blown.