I see this is more complicated that I ever tought!
Since this is going on a motercycle, I'd prefer to make something simple and rugged that's likely to survive lots of vibration.
It may be simpler to get some high power red LEDs and wire them up in strings that will operate from a lower voltage and simply use resistors to set the current.
Something like
this would be a good starting point.
They're already on "stars". These need heatsinking, so you would screw them carefully onto a metal (preferably aluminium) plate.
They're rated for 700mA, but I'd probably not operate them above 300mA (and you may not even need that much). A string of three of these will drop about 7.5V, so nominally, a resistor of 120 ohms (1W) in series with them would operate them at 50mA -- which may be sufficient for a taillight.
An 18 ohm (5W) resistor would operate a string of three at 300mA, and that would be good as a brake light.
Add 2 diodes and you can operate the same string as both a taillight and a brake light.
I would recommend that you play with the resistor values until you get a suitable brightness for both tail and brake light. For experimenting, you could get away with 1/2W resistors for the taillight resistor, and 1W for the brake light resistor. They may be cheaper and although they'll operate pretty hot, they will be OK for testing on a bench.
You probably need to mount these inside an existing waterproof enclosure. If you have room, there is no reason why you couldn't have 2 or more strings of three LEDs.
Flashing the brake light is very visible but also very illegal.
The ones I've seen go on-off-on (and stay on) as the brake is applied. They don't flash continuously. But yeah, probably illegal.
I buy these LED's from China, Ebay, so I guess the contraditory information it's explained...
And also sometimes overgenerous specs. Try to run the LEDs at half power or less. It will help. It's actually not so important in this case because the brake light is not on for long periods.
The use of LED's I thought it was best since the motorcycles produce very few energy, compared with cars, and also the incadescent bulbs release much more heat. And I need something small, sturdy, visible and energy eficiente.
What are the ratings for bulbs used as tail and brake lights on your bike?
You'll find that 1W of red LED is the same as maybe 8W to 10W of incandescent. If you use a red LED, it's even better because almost all the power is in the colour you want (could be a factor of 2 improvement)
With 1 string of LEDs, I have suggested 0.375W and 2.25W which might be comparable to 6W/40W. But this is just rule of thumb. You need to do the comparison yourself.
This suggests that these figures are in the ballpark.
The brightness will vary as the battery voltage rises and falls. This is another reason to de-rate the LEDs. I did my calculations for 12V (perhaps I should have used 13.8?). If the voltage rises to 14.5, the LEDs will be a lot brighter, but will not be operating over their specified maximum ratings.
If you have any other idea for this please, I appreciate!
I hope the above helps.