Wilton Posted August 5 Report Share Posted August 5 Got a leak in my master cylinder hose, so I took it apart to replace the hose and found this fairly soft black rubber item inside the master cylinder. It did was not attached to the lid so when I removed the lid it remained in the master cylinder. There doesn’t seem to be any hole in it so I’m pretty sure that the fluid goes into the master cylinder before this does. I can’t see any purpose for it, it just seems to reduce the volume of the cylinder! What’s it’s purpose and do I need it in there? Thanks Quote Link to comment Share on other sites More sharing options...
Glayne Posted August 5 Report Share Posted August 5 Off the top of my head I believe I have read that it is there to separate any air and the hydraulic fluid from absorbing moisture from the air. Fluid in first, seal in last. Feel free to correct me if i'm wrong... Quote Link to comment Share on other sites More sharing options...
Wilton Posted August 5 Author Report Share Posted August 5 Ok that certainly sounds plausible, but it seems to take up so much volume, unless of course it is meant to be more collapsed?? Quote Link to comment Share on other sites More sharing options...
rotors7 Posted August 6 Report Share Posted August 6 Yes, you just collapse it into itself. Quote Link to comment Share on other sites More sharing options...
richt Posted August 6 Report Share Posted August 6 (edited) You say you had a leak, so as the fluid level dropped it has sucked the rubber boot down into the master cylinder. Don't worry that's how it's meant to work. Just collapse it back into shape then put it back in the top of the reservoir and then put the cap back on. Edited August 6 by richt Quote Link to comment Share on other sites More sharing options...
lemur Posted August 6 Report Share Posted August 6 Brake fluid is hygroscopic (absorbs water from the atmosphere) the brake master cylinder needs to be vented to atmospheric pressure. The rubber bladder separates the brake fluid from the water. If it was wet on top when you opened it, that is water and it's working. Quote Link to comment Share on other sites More sharing options...
Wilton Posted August 6 Author Report Share Posted August 6 That’s great thanks for the advice chaps. I can put it all back together now knowing how it works 👍👍👍 Quote Link to comment Share on other sites More sharing options...
richt Posted August 7 Report Share Posted August 7 If you've not had the pleasure of bleeding a GasGas rear brake before, I predict this might be your next question!🤣 It really is a huge pile of fun! Quote Link to comment Share on other sites More sharing options...
Wilton Posted August 7 Author Report Share Posted August 7 Ok so I’ve not started yet….so can you give me a clue / guidance / any helpful tips! I was just planning on pumping new fluid through the system in the normal way by sequentially loosening the bleed nipple, depressing the pedal, tightening the nipple and releasing the pedal….all whilst keeping the reservoir topped up. Will that not work or is it just a pain cos it’s fiddly? Thanks in advance! Quote Link to comment Share on other sites More sharing options...
lemur Posted August 7 Report Share Posted August 7 I do the exact opposite; empty the reservoir of old fluid, fully retract the pads to force as much fluid out of the calliper as possible and use a great big plastic syringe to force fresh DOT fluid in through the bleed screw, clean up and discard any old fluid that get pushed up into the reservoir, then top up the reservoir with fresh fluid. Done correctly you won't be introducing any air into the system at all and bleeding might not even be required. Quote Link to comment Share on other sites More sharing options...
Wilton Posted August 8 Author Report Share Posted August 8 Hi Lemur. Thank you for that, I’ve never tried it that way before! I’ve got a big syringe though, so I’ll give it a go… Quote Link to comment Share on other sites More sharing options...
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