rear brakes will not stay pumped up!!!

[steveo3318]

ok after changing out the fluids i mixed up, my rear brakes will keep bleeding down, they are not loosing fluid. only acting like the are getting air in, i have bleed them six times and the keep hitting the floorboard after a few miles, i can also pump them fast while riding and they will get firm again for a few minutes. after i bleed them they are very firm. is it possible i have yet to get all the air out or they are somehow getting air in or i have ruined something by putting less than 100 miles on them mixed with dot 3

 

[glider]

Sounds like the master cylinder is loosing the seal on the cups that keep the pressure on the system. If it's not any trapped air in the system the cylinder should be rebuilt.

 

[biscuit]

I concur!IF it's not trapped air and you're sure you've bled the brakes correctly,then my money is on buggered seals.Fortunately,you can easily check these.

Grab a hard and instant footful of brake.This instantly forces the seal against the sides of the bore of the master cylinder and provides a good seal,and will be evident by resistance to movement.(ie,your brakes will feel normal).However,this will soon bleed off.
Next,SLOWLY squeeze the brakes on.If your seals and cylinder bore are no good,the pedal will depress all the way.By SLOWLY squeezing the brakes on,you do not suddenly expand the seal and force it to make contact with the cylinder walls .This allows brake fluid to seep past the seals and will provide proof that they're stuffed.
Good luck!

 

[steveo3318]

sounds great! i will try this shortly today sometimes, anyone have any idea about how much the rebuild kit is for the master cylinder and if its something i can rebuild or need the shop to do. also what about the actual brake itself and its seals, coulld they need replacing as well, i can't find a drop of fluid anywhere on the bike. sometimes after bleeding they will stay firm for hours of riding then all of a sudden they are gone, its only the rear brake, it really flipped me out when one time they were gone then a few minutes later i hit them just to see and they were back. that was without pumping them. Also when i bleed it in the garage and pump it or test it, they stay firm for days until i take it for a ride, then shazzam they disappear. thanks and sorry for the long post. i am going to make dang sure the air is out one last time lol:bigsmiley30:

 

[glider]

They usually run less than $20.

This is basically what you are looking at.

 

[steveo3318]

well i pumped them up for the last time earlier this afternoon, for some reason i bleed them out several sevarl times and rode it for about three hours and its just as firm as it was from the factory. guess i had a touch of air in them. thanks for all the help and i may still rebuild the master just for good faith thoughts. lol