In some cases (joystick/pedal model) is need do the inverse procedure for assign an axis.
Press the right brake on pedal and HOLD, select the command "Right Wheel Brake", when the assignment window pop-up, release the pedal. Same for lef brake.
In Battle of... series, "toe brakes" work for Luftwaffe, American and the Russian i-16 aircraft.
For all other Russian planes, British and Italian planes, you need assign a joy button or key for "Wheels brake" and use this together with rudder movement for achieve "differential brakes" - brake only right or left wheel, as IRL in this planes. Or if you are lucky owner of VKB MCG or VPC T50 grips, use their brake lever.
Don't try "cheat" (like in other flight games) and assign right and left brakes of pedal for this "Wheel brake" command, because this will mess brake in planes that use "toe brakes" system.
The reason is that for benefit (intentional or accidental) of "twist rudder" joysticks, this "differential brakes" work for all planes of the game, since normally "twist rudder" stick users don't own rudder pedals.