Let me get this right. If there is a high to the input, the pulse detector goes high. This starts the first counter. The inputs to U11 are both high so that turns on all lights at once. If there are no additional pulses but the input stays high (brakes on) the sequence start counter U3 doesn’t get to the second output Q1 and the sequencer clock never starts. If there is another pulse, the high out from Q1 is fed back to the input and stops the counter from incrementing. U11 output goes low so all the lights don’t go on at the same time. The sequencer clock and U5 make the lights blink in sequence.
Looks like it may work. Circuit has to be duplicated for the other side, right? I thought this was kind of complicated but now I realize you put a lot of thought into it and it should work.
The delay for the turn signals is not really a problem. First all lights are on one side then it starts sequencing. That’s OK. Main thing is no delay when brakes are put on. I found a definite difference between the response time of LEDs and regular filament bulbs, apparent when using an electronic flasher driving both in parallel.
Thanks for the idea, now to find parts to try it out.