Service letter SEL-32-05 from Textron Aviation states that leakage has been reported from the outlet port of the brake master cylinder for certain Cessna 172S, 182T, T182T, 206H, and T206H models. The letter states that this leakage may be caused by an incorrectly installed O-ring or installation of an O-ring of the wrong size. In either case, the leak may result in decreased effect of braking.
The letter is informational only and does not give any required actions. If a leak at the outlet port is found and it is determined the O-ring must be replaced, O-ring MS28778 should be used and proper fitting should be confirmed.
Download the service letter and check if your Cessna may be affected.