Stop Light Switches for Hydraulic Brakes

Why do Harley-Davidson Big Twins use hydraulic brake light switches?

Because of the hydraulic rear drum brake – a setup that doesn’t use the usual rod or cable to activate the brake shoes, but instead a hydraulic cylinder under the drum that spreads the shoes when you hit the brake pedal.

And why do Big Twins with Panhead and early Shovelhead engines use hydraulic drum brakes in the first place?

It’s because of the rear swingarm suspension. More precisely, to make sure the rear brake actuation isn’t affected by the up-and-down movement of the wheel.

Of course, the hydraulic brake light switch isn’t limited to drum-brake 4-speed Big Twins with round swingarms. It fits most models without ABS, including Softails, Touring models, Sportsters from 1979 on, Buells, V-Rods, and more – even if they’re running disc brakes.