Would someone with DT360 check to see if their brake light dims when the indicators flash please.

What wattage tail bulb are you running. Some are quite high wattage on the stop filament. We used to run as low as 10/3 wattage but think normal was 18/6 back then. But I've seen 23/8 too which draws a lot when expecting flashers & brake light to operate together.

My globe is a 21w, standard is a 17w, do you think this could be the problem?

Lets see what other say before head of in the wrong direction.
I guess by "74 the charging system was getting better.
Are you talking motor running or no motor.
I just had similar issues with my little 100 single but had a very bad earth for the battery through a painted battery box it was earthed to plus the long through bolt that held it to the frame was very rusty.
I'm not saying that's your issue but keep it in mind.
If 17watt stop light is std then i guess try that or lower depending on the laws of where you are.
Are your flashers using std bulbs & relay or higher wattage & relay to suit those bulbs.
Battery needs to be in top condition too of course. Good quality battery.
I'm just looking on CMSL & i see the flasher bulbs are listed at 18watt & tail stop light at 25/5.3, [25 stop light] so i don't know anymore.
I guess the system is designed to support those wattages in a perfect world.
I'm working myself into a corner. Cheers. Ps, i'm sure someone will suggest those new fangled bulbs that put out a bright light with less battery draw.
Sounds like a good plan. I'll look & learn--well, i'll look but too old to learn new tricks.
Yamaha bulb part number below i googled.
www.megazip.net/zapchasti-dlya/yamaha/bu...25-5-3w-1M1845146000
www.cmsnl.com/yamaha-dt360-1974-usa_mode.../partslist/E-10.html
www.cmsnl.com/yamaha-dt360-1974-usa_mode.../partslist/E-12.html
www.cmsnl.com/yamaha-dt360-1974-usa_model8481/partslist/

1972 DT2 250, my brake light dims with the flashers