Thinking back on the causes for leaks......
Brake pipes are flared and the flared part is designed to mate against the matched fitting.
Years ago I had a smell of diesel on a car and it was cured by releasing and refitting the fuel pipe inlet so the mating surfaces reseated correctly.
My guess is that your car has issues with the mating of the pipes to t-piece. Matching up correct threads is a possible cause as it can prevent proper mating contact. So....check that, check the condition of pipe flares and t-piece. As the pipe connector is not removable due to the flared pipe check that to get a matched t-piece.
I upgraded my front callipers and the brake lines used banjo fittings to connect. These used copper crush washers for sealing as no flared pipes.
