Hi there,

 

  I'm having some problems with my front brake that I just can't seem to figure out and am hoping someone here can help.  I have a 2014 Sherco with a Braktec caliper and master cylinder.  Here is the issue:

The front brake is soft, but not always.  I tried regular bleeding procedures by pumping the brake, holding, releasing bleed valve, tighten, repeat.  I just put on a brand new caliper, hose and hardware connecting the hose to the caliper.  I reverse bled the system taking care to not have any air in it.  Once I did that, things were still quite soft and went back to regular bleeding process.  That helped quite a bit.  I did the bleeding process for a solid 20 minutes and the brake felt nice and firm.  Every time I pumped the brake, held, then released the bleed valve, I could see air bubbles coming up through the master cylinder upon opening the bleed.  They were very tiny.  Again, did this for about 20 minutes and it never stopped.  Eventually I put the cover on the MC.  Now the brake is spongy, to not working at random.  If the bike sits for a few minutes then I pull the brake, it will bottom out giving no braking power.  After a handful of pumps braking power will come back.  Sometimes if I'm hopping on the bike with the lever pulled in it will work at first, but a 10 seconds in the braking power goes out.  

 

Any thoughts?  More time bleeding?  Perhaps a rebuild in the master cylinder?  

Any help here would be extremely appreciated.  I'm growing tired of not being able to ride.

 

Thanks,

  Seth

 

 

 

Rebuild the master cylinder. 

Sounds like you need new seals for the MC

Great!  Thanks guys.  Ordered a rebuild kit and will let you know how it goes.

 

  The rebuild kit is the first fix. Many of these have been problematic over the years. Some repair them, or replace with another M/C.

Thanks for the recommendation.  I rebuilt it, and backfilled the system from the caliper.  Immediately the fluid rolled up into the MC with far less pressure than what I was using before.  Sealed her up and the pressure is back to normal.  Thanks again.

 

 

I am interested what have you used for back filling. I tried 150mm syringe and could not get enough of pressure from it for the oil to go through

Edited March 17, 20197 yr by Petr
typo

I used a 60mm syringe.  Before I rebuilt the MC the pressure was so intense and it filled so slowly I couldn't tell if it was working.  Once I rebuilt, it still took some pressure but you could see the reservoir filling pretty quickly.  If it feels like the syringe is going to explode, something else is probably going on.  I also had to clean up the bleed nipple a bit.  Pulled it out and the hole in the bottom portion was kind of mangled.  

 

Use a vetenary syringe there a good size to hold and they hold quite a bit of fluid 

After bleeding the brake so many times I decided to change the seals on M/C front brake. Unexpectedly easy job with extraordinary costs for a few pieces of rubber (£21!). It is now better and still a space for improvement ones everything gets seated.

When you did back fill, did you operate the handle at all?

I didn't operate the handle at all during the back fill.