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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
    Commented 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
    Commented May 17, 2022 at 17:16
  • I don't understand why you could not simply put in a "T". Why would backflow cause any issues? With a single lever you put pressure on the whole system when you actuate it, and nothing bursts... In fact every bit in a hydraulic system is built to work under lots of pressure.
    – MindDBike
    Commented May 18 at 5:50

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The GRX BL-RX812 Sub brake levers (interrupter levers) work because the fluid input port from the primary lever is blocked by the master cylinder piston when the sub lever is closed. The MC piston essentially acts as a valve stop for the hydraulic input side as soon as the lever starts to compress.
Very Rough Diagram of MC Piston valve movement I could be mistaken, as I'm nobody's engineer, but I don't think this action can be labeled a check valve, because it wouldn't stop fluid back pressure if the lever was not compressed.

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    This confirms that both levers cannot be used at the same time, I believe. Commented May 12 at 22:10
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    I don’t believe that you can use both levers, no. More importantly, Since the levers both have the same level of power and function, there is no benefit to using both levers.
    – zenbike
    Commented May 13 at 19:01
  • Because you would have both the main lever and the interrupter lever on the same side of the bike, I am having a bit of trouble seeing a situation where you'd want to pull both at the same time, even if the effect was additive.
    – Weiwen Ng
    Commented May 16 at 17:49
  • On the hydraulic version, it wouldn’t make sense. People used to do it on the cable version because they never worked well in the first place.
    – zenbike
    Commented May 18 at 23:35

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