Welcome to the Team Don!
Yes the turn signal being bad can affect your brake lights. Any time you are dealing with an electrical issue you are dealing with a ground issue because GM chose to use the steel body as a ground instead of running a separate ground wire to each device. Rust, paint over spray, poor termination all over time have created grounding issues with these older cars.
The dimmer circuit of the light switch should not be in any way be associated with the tail lights, or your brake/signal lights.
There are wiring diagrams you can down load for free to trouble shoot your circuits if you have a ground wire going back to the battery to test to see if a bad ground is the problem before you get fully involved. You will also need a multi-meter as well as a testing probe. I find a growler that makes a noise when a circuit is powered handy to find opens in wiring.