How to fix my Oldsmobile cutlass supreme?

Load test your battery and replace it if necessary. While you're at it check the charging system to make sure it is functioning properly. If that doesn't help, then I'd suggest a fault of some sort in the wiring harness that draws power or interferes with some other electrical component when you apply the brakes.Examine the electrical system as closely as you can looking for worn or pinched wires, loose connections etc. In my car I discovered the plastic signal light flasher under the dash had collapsed and was creating some sort of an electrical drain that caused starting problems. Once replaced with a new metal one the car ran like new again. If you're not up to the task, find a shop that specializes in automotive electrical and spend a few dollars having them go through everything. It will be money well spent if you plan to keep and enjoy the car.