Left side brake woes

First thing is ground. If both park/tail lamps are equal brightness, thats a good indicator of grounds and proper bulbs.
The same green wire is used for brake and turn. Also supplies the same element in the bulb. Both brake lamps should also be equal brightness.
If all about the green wire... If turn is bright and brake is not, or vise versa, the fault has to be in the signal switch contacts. The yellow wire at other rear lamp serves the same duty. So you could compare L&R operation in diagnosing if necessary. Left works fine , right don't, signal switch.