Shimano road disc left-hand lever diaphragm ST-R9120 Y0C578000

The diaphragm (membrane) Shimano Y0C578000 for the left-hand lever ST-R9120 is a small sealing component located inside the body of the hydraulic road brake lever.

It plays a vital role: it compensates for changes in the volume of the fluid ( Shimano mineral oil) in the master cylinder whilst ensuring the system remains sealed.

Part in question

• Name: Left-hand lever diaphragm / membrane
• Shimano part number: Y0C578000
• Compatibility: Shimanohydraulic road levers
  • ST-R9120
  • ST-R8020 / R8025
  • ST-R7020 / R7025
  • certain compatible GRX shifters

What is this diaphragm used for?

The diaphragm acts as a balancing bellows:

• it absorbs the expansion of the fluid during temperature fluctuations
• it prevents overpressure in the lever
• it ensures the internal reservoir remains sealed
• it protects the system from air and moisture ingress

If it is punctured or deformed, the following may occur:

• oil leaking from the lever
• a gradual loss of brake pressure
• a spongy feel to the lever
• bubbles or seepage during bleeding

Replacement

Replacement is generally:

• simple but delicate (opening the lever cover)
• requires checking for leaks after reassembly
• often recommended to be carried out in a workshop if the system has been contaminated

Workshop advice

On Shimano ST-R9120 levers, a damaged diaphragm is a common cause of internal leakage. In some cases, a simple clean is sufficient, but if the part is cracked, it is better to replace it rather than attempt a temporary repair.