This the what the law requires for UK registered motorcycles.
Motorbikes first used before January 1936 don’t need to have any brake lights fitted at all.
Bikes first used before April 1986 must have a brake light that works from at least one of the braking systems.
Bikes first used after April 1986 must have a brake light that works with both of the braking systems.
This what the Road Vehicles Lighting Regulations 1989, Schedule 12 says:
“Every stop lamp fitted to… a solo motor bicycle or a motor bicycle combination first used on or after 1st April 1986 shall be operated by the application of every service brake control provided for the use of the rider.”
The “Motorcycle and Side car Testers Manual” covers the MOT requirements for motorbike brake lights in Section 1.3. It states:
“Exemptions: For this test if a stop lamp is fitted, it must meet the requirements of this inspection, but need not be fitted to a machine which:
a. cannot exceed 25mph, or
b. was first used before 1st January 1936, or
c. was first used before 1st April 1986 and which has an engine capacity of less than 50cc.
Machines first used before 1st April 1986 must have a stop lamp that operates from at least one brake control.”
So, in effect if your bike is over 50cc and even though it was originally built and supplied before 1986, the regulations are retrospective, and a brake light is a legal requirement.