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It has come to my attention that interrupter-style levers are available for hydraulic brakes now.

By interrupter lever, I mean those levers which can be mounted to the same cable on an alternate location on the handlebars. In other words, you can have two brake levers, both connected to the same brake. These are popular with cyclocross bikes. There are hydraulic equivalents, see for example the Shimano BL-RX812.

For cable-actuated brakes, these levers are spliced into the same cable used for the primary brake. For hydraulic brakes, I don't understand how they work.

Normal hydraulic brake levers don't stop fluid flow back into the lever. We know this because when you replace the brake pads, you press the caliper pistons back into the caliper, and fluid will simply flow back up into the lever reservoir. So, an interrupter lever can't simply apply pressure to the same brake line. In the same way that you could not simply "tee" together two levers to one caliper. If you did, fluid would flow back toward the other brake lever (maybe bursting it) rather than pressurizing the brakes.

It is possible that the interrupter lever itself has a check-valve incorporated, or otherwise blocks the return path to the main lever when it is actuated. In that case, it would not be possible to actuate both levers at the same time. Is this how they indeed work? This is a little bit worse than the cable-actuated version, in which both levers can work at the same time, but I suppose very few people care about that capability.

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  • With cable actuated interrupter levers you "could" use both levers at the same time. But in practice that wouldn't really be done as you would have to reach across with your opposite hand to use the interrupter lever while using the proper hand on the main lever. This doesn't sound like anything that someone would ever do while riding. Also, squeezing the main lever wouldn't add much if any additional braking as you would mostly just be taking up the slack in the cable between the main lever and the interrupter lever.
    – Kibbee
    May 17, 2022 at 16:41
  • I think that it's likely you are correct about the check valve, which would work in much the same way that the reservoir is isolated from the brake line when the main lever is actuated. Unfortunately I don't have one to test but it is hard to imagine that it could work any other way.
    – Noise
    May 17, 2022 at 17:16

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