Brembo bleed nipple sealing issues

Hello there -

I have an issue with a Brembo rear caliper. It's on a Monster 620, effectively, but I don't think that's important because this question is more general than related ti any specific bike.

Using a vacuum pump to bleed the line, it's apparent that there's an air leak and I believe it's at the point where the bleed nipple seats into the caliper.

Has anyone come across anything similar?

I have never seen this, so if you've experienced it, is it common to have to replace a nipple, or does a leak more likely indicate that the surface of the seat in the caliper is unserviceable??

Answers on the back of a postcard, please...

Are you quite sure the vacuum pump/nipple connection is not the problem? Sounds likely to me

Thanks for your response and yes, that's quite possible.

Here are the symptoms -

the system was working, then without any loss of fluid, stopped working - there is no resistance at the pedal
the system won't pressure-up through the usual pumping-the-pedal method
the reservoir is full, and the level does not decrease with pumping
using the vacuum, bubbles stream from the nipple up the clear tube towards the vacuum pump (even after 10 minutes, they continue to stream so the bubbles I am seeing are not merely air that has been trapped in the system)
closing the nipple tightly still allows a stream of (smaller, but still quite a flow) bubbles up the pipe

So yes, with a closed nipple the air could easily be coming through the tube-to-nipple connection - that would fit the observations - but that connection seems very tight and even if I take off the tube and nip up the nipple, there's air coming into the system somewhere...

I know that in order to solve these things you often you have to come up with a theory and keep going until evidence disproves it.. then think up a new theory. But right now, this one still seems to be the best fit, as far as I can see.

But watch this space...

Try putting a very thin smear of rubber grease on the seat of the nipple and don't over tighten it.

Thank you - I will try that and report back.

P

It's likely that the bubbles you're seeing are air seeping down the loose thread of the open bleed nipple and up the pipe. If the nipple wasn't sealing in the seat then you'd have a fluid leak, simple as that.

If you have no leak and you have no pedal pressure then it'll either be the master cylinder seals or a ruptured structure in the brake line allowing it to expand.
It's very unlikely to be the brake line as I happen to know it's a braided line and it would need to literally balloon up to give no pedal pressure.

That pretty much leaves the master cylinder I'm afraid

The Other PaulG said:

Thank you - I will try that and report back.

P

Click to expand...

I meant to say put the grease on the thread ..... assuming it is the nipple

Thanks fellas - I am going to do some testing.

I have some new nipples in the post (!) and if it's not that, the price of a replacement master cylinder's not as bad as I had imagined (c.£55) so let's see!

P

I have a mityvac for doing my brakes and I to see what appears to be air in the system and that has ALWAYS been the pump pulling air past the threads, its not enough for very much fluid to come out if bleeding but when pulling through with the vac pump it always seems to pull the air in.
If the brakes work and then dont, and pumping the leaver/pedal does not build up pressure then the master cylinder is likely to be the issue.

I recently had a similar issue with my KTM990 in that the brakes would be fine and then not, in my case it was air getting in past the seal on the sight glass, whilst on "tour" I used super glue to seal it and then replaced when I got back.