Hydraulic Brake Pressure Switch by Bonamici

Many race-oriented brake master cylinders and rearsets do not come with provisions for brake light switches, as most race bikes do not utilize brake lights. However, many street riders want the performance of a racing brake system but still need to maintain the rear brake light function.

This universal switch allows rearset users to operate the rear brake light. This all-in-one unit replaces the banjo bolt that connects the rear master cylinder to the rear brake line. You simply replace the bolt with this assembly and splice the wires into your existing brake light switch. The pressure inside your brake system then activates the brake light.

This piece is sometimes called a Hydrostop as well.  If you see the word hydrostop, they are referring to a pressure switch.