A couple of weeks ago, I was working on the Master cylinder for a SI 330, and the brake light switch was broken. The original switch (pictured below) has a metric thread that is no longer available. So I made a modification that readily accepts a standard brake light switch with a tapered pipe fitting.

I chucked the original switch in my lathe and cut out the switch internals. The switch is pretty simple with a rubber diaphragm that pushes a spring loaded disk into two contacts that completes the circuit of the switch. The failure points must me at the rubber diaphragm.

I cut the shoulder of the original switch down so it would not interfere with the mounting of the new switch and drilled out the center so I could put pipe threads into the old switch.

The new switch threaded perfectly into the old switch shell. I’ll seal the pipe threads with some Teflon tape and reinstall this set up on the master cylinder with a copper washer on the side of the old switch. The switch I’m using is made my “Painless Wiring,” and is a “low pressure” switch. I wanted to try this out because one complaint I have with other switches I’ve used is the brake lights won’t come on unless you were stepping moderately hard on the brakes. We’ll see if this switch is more sensitive.