I had to drill out the rear brake reservoir screws on my '98 Suzuki GS500E as they'd rotted but in doing so I managed to snap the drill bit. Carelessly, the top of the reservoir was off by this point and the inch or so of drill bit may have ended up going into the reservoir and down the pipe that connects to the master cylinder.

Since the fluid is due for changing can I simply detached the master cylinder, invert it and drain out the fluid and confirm the drill bit is either in there and remove it, or not there at all. There is no ABS or anything so I hope I can simply refill, bleed a few times and all will be well but I'm not sure how well master cylinders react to this sort of treatment? Is there anything to watch out for when draining down a master cylinder on a bike?

  • Not sure if it's different for motorcycles (it shouldn't be), but you should never empty the master cylinder if you can help it. The reason is to avoid introducing air bubbles that get trapped in the brake lines, causing spongy brakes. The bubbles can be quite a chore to get rid of.
    – Zaid
    Mar 29, 2017 at 13:04
  • I'm hopeful that because its tiny I can fill most of the unit and slosh the fluid around prior to refitting. Apparently the replacement interval on the lines is every 4 years (gstwin.com/every_4_years.htm) and i'd be amazed if replacing those didn't introduce any air?
    – tobyd
    Mar 29, 2017 at 13:11
  • That's true, but I've never known anyone to replace their brake lines, except with stainless steel upgrades.
    – raydowe
    Mar 29, 2017 at 13:16

1 Answer 1


You have two concerns:

1) Introducing air into the system.

Now that the reservoir is already open, you should be replacing the fluid entirely anyway. You may be able to remove the reservoir and hose, empty it, replace it, and not get any air into the system. Ideally you want to try your best to avoid air getting into the system in any circumstance. Regardless of how much air gets in, you've gone this far, and replacing the fluid needs to be done periodically anyway. This means bleeding the brakes, which will force out the air along with any old fluid. Just make sure you do your research so that it's done properly and all the air is removed.

2) Metal contaminating your master cylinder.

It depends on the bit and how it broke. If it's just one big chunk of the bit and you can get it out, then that's fine. My concern would be ensuring any little pieces don't make their way into the master cylinder and prevent it from operating properly. Depending on how exactly the bit got in there, and how confident you are that it's only one big piece, you may want to consider disassembling the master cylinder and giving it a clean.

  • motorcyclespareparts.eu/suzuki-parts/motorcycles/gs500e-1998/…. The bit was a 3mm section, entering (if it did at all, it may have pinged off into nowhere) down pipe 17. End of drill bit left in drill had snapped off where the shank becomes the non-shank so I estimate around an inch is missing - very possibly enough to get past part 7 and into the body of the master cylinder.
    – tobyd
    Mar 29, 2017 at 13:33
  • Possible, although I'd be surprised if it could get into the master cylinder. The valving (#2) should mostly fill the cylinder. My concern would be any small secondary pieces that came off when the bit broke.
    – raydowe
    Mar 29, 2017 at 13:42
  • I see what you mean, I was concerned that the bit would get between the plunger and the end of the cylinder and prevent it activating, but I can see how thats unlikely. Cumulative grinding from rogue shards will wreck this so I'll disassemble - thanks for pointers.
    – tobyd
    Mar 29, 2017 at 14:28

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