Parking brake

My Firefly came with a button on the right brake lever that you could press to hold the brake for parking.  This feature is great, because you can stand the Firefly up in a corner when not in use, or set the brake at a traffic light and use both hands for your phone, to dig in your bag, etc.  This is also a safety feature, because once the parking brake is engaged, accidental throttle activation has no effect.  But after about 500 km (300 miles), the button broke, and all of the parts scattered around!

What's left of the original brake-hold button.  The spring is missing and the bottom head broke off.

So I contacted the manufacturer (Rio Mobility), and they wrote that others were having the same problem, and their "solution" was to remove the brake-hold feature!  No warranty fix is possible.  I can sympathize, since the company probably doesn't want to be in the brake-lever engineering business.

But I have an affinity for mechanics, and giving up on this little bit of sweetness was not so satisfying.  Clearly, the button breaks because every time you release it, a spring causes it to pop up and smack into the brake housing with large peak forces.  The poor button can't withstand the repeated violence, and the part fails.  This seems like a design flaw, but most users of these brake levers are on bicycles or recumbents, and they won't use the brake-hold enough to cause failure during the warranty period.

After some experimenting, I ordered a replacement brake lever (Sun BL46K) for $13 that includes a brake-hold button.  This has a slightly higher quality feel than the original, but the brake-hold mechanism appears to have the same not-so-good design.

Fortunately there is a simple fix!  You can take a small cushioning washer like McMaster 93650A100 and stretch it over the bottom of the brake-hold part like a rubber band.  Now the brake lever can last a long time, because the peak forces that caused the part to fail are much lower.  Feature restored!

The red oval shows where the cushioning washer (which is transparent) was stretched over the bottom of the brake hold button.

Update (October, 2021):  Over 6000 km later, the brake-with-rubber-washer works as well as ever.  This page will be updated if a problem develops.