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I managed to successfully bleed a Shimano hydraulic brake but used an improvised kit instead of the official Shimano one. Since I used a regular syringe instead of the funnel+plunger system which may guarantee a certain oil level in the master cylinder reservoir when used properly, I was wondering if the oil level is the master cylinder reservoir is critical.

  • Which brake system? Do you have a link to the official instructions? – Argenti Apparatus Jan 12 at 17:51
  • It shouldn’t matter. The Shimano funnel is convenient with the plunger but end of process you have a reservoir of oil above the lever body. If majority of air is out it’s the same thing. – Warren Burton Jan 12 at 18:29
  • @Warren Button: I was wondering more if the problem is with too much rather than too little oil. With motorcycles there's a minimum AND a maximum fluid level. That's so that the fluid has room to expand in the master cylinder in the free travel portion of the lever. – user3671607 Jan 12 at 20:08
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The system is designed to have the lever bleed screw displace some oil as it's reinstalled. In other words, the lever end wants to be completely full at the end, and there is not supposed to be an air gap there

In the too little fluid direction, there is some tolerance for an air gap before symptoms begin, but ideally you wouldn't have any. If by chance you finish the bleed and find out there to be a small air gap at the lever, if it's small enough then dripping a little oil in may be reasonable, but usually there's none when you pull off the funnel so if someone does wind up with one, it would first make me worry that something went wrong in the bleed.

If one doesn't have a funnel, then most of the time, doing the "gravity bleed" part of the procedure the same way as with an older Shimano brake works fine, i.e. by dripping in more fluid in the screw hole as the fluid feeds into the catch bag/bottle at the other end. Doing this with road levers would be clumsy at best though, as they tend to often really need to have their angular position changed during this step to free up air bubbles, as per the official instructions.

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  • Appears I've inadvertantly downvoted this answer seven hours ago (falling asleep). Won't let me correct it. Apologies. – Jeff Jan 13 at 15:27
  • Re. "dripping in more fluid in the screw hole": for your (and anyone else's) information, a regular syringe screws perfectly in the Shimano oil port, which makes things much easier than dripping oil in the hole and risking introducing air past the master piston. – user3671607 Jan 13 at 18:49

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