I can see answers for this question with cables but cannot find an answer for hydraulic brakes. I am looking for a 31.8mm clamp hydraulic disc brake lever for my fixed gear commuter, and have seen the shimano grx BL-RX812 sub lever. Does anyone know if this be used as a primary lever?

1 Answer 1



  1. It has two ports, the one normally connecting to the main brake lever needs to be blocked.
  2. If you somehow manage to block it. It's reservoir probably wouldn't be large enough to adjust for brake pad wear, as BL-RX812 relies on the reservoir of the main brake lever.
  3. You would need to block the port using some kind of diaphragm to allow pressure equalization between the reservoir and ambient pressure.


The animation is from https://commons.wikimedia.org/wiki/File:Hydraulic_disc_brake_diagram.gif

Instead of the reservoir the fluid inlet and compensating ports of the BL-RX812 are connected via hyraulic line to the outlet of the main brake lever. The only way to make them work independetly is to add some kind of external reservoir.

It will be way easyer to just get some 22.2mm handlebars and normal flat-bar levers.

  • Thanks for taking the time to answer and sharing your knowledge. Apr 12, 2023 at 18:36

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