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I want to distance the current issues from the issues that are no longer applicable.

Gone is the LED bulb in parallel with the ignition coil, let's not look back at the failed attempt to rectifiy the AC for use by the DC LEDs.'

I believe I have my AC bike wired to stock.

I am using a 6volt dual filament bulb in the taillight rated at 18W brake filament and 6W running light filament.

The good news is that the running light is nice and bright, I would say even at idle it's decent.

The bad news is the brake lamp filament which is ipowered by virtue of being in series with the ground circuit of the coil is ineffective. It glows but does not add anything with regards to intensity to the rear lighting,

What I am saying is that it appears that I have no functional brake light.

As Bob points out there are other threads about this and I am going to go digging for possible solutions now. I tend to think I could survive a dual-filament of reduced power, something like a 9W high filament and 3W low filament so as to be able to power the brake light and have it visibly overpwer the running light when braking. Howeverm I am sure such a bulb with a 6V rating does not exist,

It occurs to me that since the running light is decently bright and since the heart of the issue is to inform the driver behind me that something is beginning to change on the motorcycle in front of him, that I might think about implementing a flasher that would quickly and repetatively flash the running light as a sort of psuedo-brake light. But for now I will put the idea in my back pocket and pursue a convention two intensity solution.

Bob; you alluded to there being some remedies for this, and that perhaps they were covered in older posts, I went looking but couldn't find anything save for some vague reference to using a diode to perhaps halve the power consumed by the ignition coil thus potentially freeing up some power for the brake lamp?

If you point me at a prior thread or give me some key search word I will refer to the prior post - I just can't find one that specifically addresses a solution.

Jim

" The good news is that the running light is nice and bright, I would say even at idle it's decent. "

____ Assuming you mean the tail-lamp's 'taillight', then that seems as normally expected (providing that the headlight's wattage is kept no stronger than 25w).


" The bad news is the brake lamp filament
is ineffective. It glows but does not add anything with regards to intensity to the rear lighting, "

____ It seems you've personally learned that which other owners of such have known-of concerning the performance of the old Ducati-rigged AC brake-light circuit,, as that which you describe concerning the performance of your tail-lamp's brake-light, seems about as normally expected (during lower than average RPMs).
However at above normal running RPMs, your brake-light ought to perform very much like a normal brake-light,, that is, lighting-up rather quickly (instead of GLOWing-up), and also cause the tail-lamp to become at least twice as bright (as the tail-light alone). _ And if not, (while at higher RPM),, then it would seem that yours could have something-else further reducing the brake-light's performance as well.
The chosen taillight-bulb could also have some effect,, as I recall there being more than one 6-volt bulb to choose from back then, I think there was a 3/18CP and a 4/21CP, and the weaker CandlePower bulb is the best choice for the non-battery powered models.


" Bob; you alluded to there being some remedies for this, and that perhaps they were covered in older posts, I went looking but couldn't find anything save for some vague reference to using a diode to perhaps halve the power consumed by the ignition coil thus potentially freeing up some power for the brake lamp? "

____ My vagueness has been due to being left in the dark as to exactly what you have been trying to do (as you progress along) and what you actually wish to get accomplished,
as I understandably don't really wish to greatly elaborate on some particular pathway which you may happen to find to be unsuitable for your particular project-goals (at whatever stage you've then chosen).
__ While probably not being actual "remedies", it ought possibly 'help' to not depend on the 'physical' ground-connection,, and ADD a wire from the tail-light bulb's base so as to actually 'electrically' ground the B-L.circuit, by connecting the added-wire's other end to the ign.coil's mounting-bracket (where you best ought to have the white alt.wire-lead also grounded at , [if not making use of a stock/AC-type B-L.switch] ).
Cuz while the physical-ground (of the lamp/bulb-base through to the fender and then through the fender to the frame), may be good-enough for the low-current taillight,, it just may not be sufficient to also fully handle the expected higher current of the brake-light. _ (Not to mention the possibility that the two separate AC signals (to the tail-lamp) may possibly be out-of-phase and both trying to pass-through a bottle-necked* physical-type ground-circuit, [* at some point in the circuit-pathway].)
__ As for the "freeing up" of some power (from the alt.power-coil which powers the ignition) with the use of just a (single) diode,, that was with the assumption that your LED already acts like a diode for itself, so then it would seem that only one additional diode would thus-then be required.
And the goal of using a couple diodes is not to "halve the power consumed by the ignition coil" *, but rather prevent (about)- half of the available current-juice from being totally wasted (directly to ground) through the ign.points ! _ (* Keep in mind that with the magneto-type ign.system, [unlike battery-powered ignition], the power-juice which actually passes-through the ign.coil is NOT actually the very-same as that which passes-through the ign.points !)
Ya-see with two diodes ya can split-up the available AC into two separate DC.outputs -(one positive & one negative), with only one running the ignition, and the other output then freed-up to power something else (without being in parallel with the short-circuit through the ign.points).
__ So to accomplish that, the yellow alt.wire-lead has to be connected to two diodes, (with one reverse-arranged from the other). _ That way you'll then have two SEPARATE DC.outputs (with opposed polarities with respect to 'ground'),, one remaining for the ign.circuit, and the newly-created/extra power-output for whatever.
Now of these two (newly separated) outputs, ONE of them will include the (rotor-timed) pulse of DC required by the ignition, while the other/extra output will not happen to have a pulse of DC.power available (at the particular-time needed) to properly power-up the ign.coil ! _ So if the output you've chosen (to connect to the ign.circuit) then no longer allows the ignition-circuit to produce an ign.spark, then that '50/50'-issue can of-course be easily straightened-out by simply flip-swapping the two outputs (so as to then provide the ignition with the particular output which happens to include the required 'timed' pulse of alt.power) !
However, if the expected load intended for the other-output (of the new/extra DC.power-source), happens to be polarity-sensitive and doesn't happen to work (with the ignition getting the output that it requires),, then in such case as that, the yellow and the white alt.wire-leads would need to have their respective connection-points swapped-around, so that the DC.polarity then becomes flipped-over for the sake of the polarity-sensitive load,, (which would be no issue for the IGNITION circuit, because IT can work with either a negative or a positive DC-pulse, [unlike loads such as a battery or LED] ).
__ (Anyone who wishes any of this stuff to be further cleared-up, please ask and then I'll gladly provide additional details concerning what's actually going-on with this dual-rectified AC.output arrangement.)


" If you point me at a prior thread or give me some key search word I will refer to the prior post - I just can't find one that specifically addresses a solution. "

____ Way-back within our first-year, I created a thread titled: " Cure for SlowGlow-BrakeLight of "40watt" models " ,
(here's a link to it... - viewtopic.php?f=3&t=81 ).
However THAT solution involves the installation of a battery, and I've gathered (from your past lack of any follow-up interest) that you apparently don't wish to employ a battery for this project-Duke of yours.
____ Since you wish to leave-out further discussion (within THIS thread) concerning the LED.brake-light solution, I'll (eventually) go to another thread and explain THERE, as to how that related method ought to work quite well enough, without having to employ a battery.


Fun-Cheers,
DCT-Bob