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

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...

Ok that certainly sounds plausible, but it seems to take up so much volume, unless of course it is meant to be more collapsed??

Yes, you just collapse it into itself.

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, 20242 yr by richt

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.

That’s great thanks for the advice chaps.  I can put it all back together now knowing how it works 👍👍👍

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! 

 

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!

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.

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…

On 8/7/2024 at 7:34 AM, lemur said:

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.

I did the syringe trick to fill the front brake hose and then put the hose on the front master. But when I push on the syringe nothing happen, it's rock solid. Do I need to pull the front brake lever to open it? 

30 minutes ago, stou said:

I did the syringe trick to fill the front brake hose and then put the hose on the front master. But when I push on the syringe nothing happen, it's rock solid. Do I need to pull the front brake lever to open it? 

Undo the lock nut on the lever with a 10mm spanner and unscrew the adjuster screw to allow the master cylinder piston to touch the circlip and thus open the circuit to the reservoir. if that does not work you have a blockage somewhere.

Edited April 22, 20251 yr by Tr1AL

2 hours ago, stou said:

I did the syringe trick to fill the front brake hose and then put the hose on the front master. But when I push on the syringe nothing happen, it's rock solid. Do I need to pull the front brake lever to open it? 

When you remove the brake master reservoir cap and force the disc pads full width apart did that not force fluid back into the reservoir?  If not then you found a problem.

The hose is not blocked but the master seem to do nothing when I pump it. I removed everything from the master and cleaned everything and I can't see why it doen't work. I even replaced the piston and seal, I have another one new. The hole on the buttom of the master is clean and there's no leak from the master or the caliper. It's the first time I see something like that. We have our first trial of the season sunday, so I ordered a cheap chinesse master and a hose from Amazon. I'll see if I can do something with that before sunday