Hi, i know the basics of brake bleeding and have successfully changed fluid many times however never to the level i have this time. My front discs were sticking on, building pressure as i used them and getting worse for about five miles when they seized. Let them cool for ten mins and they relaxed again. I rode five or so miles using no front brake and it did the same? Odd! So i thought maybe the discs were rubbing , heating the fluid and expanding the pistons? I dont know. So i stripped the entire caliper down, cleaned every part and find the pistons are tight in the chamber due to the inner seals that stop the fluid leaching through from behind. The brakes would squeeze but not retract. Once cleaned i put back together so i have a system with no brake fluid at all. I am trying to bleed the fluid through now but none seems to leave the master cylinder at all. I open the bleed nipple, squeeze the brake, lock the nipple etc but it seems that no fluid is being drawn into the pipes at all. Do i need to fill pipes or caliper from somewhere first or is it simply that the brake lever pushes such a tiny amount of fluid into my pipes that it will take all week to fill?
So ive read some online tips and other forum posts but still confused. I took the lower knuckle off the caliper and bled the main pipe from the cylinder resovoir to the caliper and refitted it. So i know the pipes full. I forced fluid in through the nipple to back fill and then did a standard bleed by putting a pipe from the bleed nipple to a bottle of fluid and cracked the nipple, pump lever, lock release etc. air bubbles can be seen but the fluid level hardly changes. I reckon the air comes through the threads in the bleed nipple and not from the system. The system just doesnt seem to get pressurised? I read a thread about disconnecting the lever and manually pumping but i didnt understand as i have no cable on my brake lever, it just activates a fluid plunger. I cant see where ive gone wrong . I even tried a self bleeder pipe with some valve on the end, still no luck. Any advice welcome
try turning the bars from side to side whilst doing it, sometimes you get an air bubble stuck right up the top just past the banjo bolt (in fact, it happens a lot)
I remember when i worked in a garage fixing cars and sometimes the brake pipe flexi's used to die, (break down inside the rubber) and the brakes would apply but stick on. slowly the pressure would relieve but once the flexi was replaced the problem was solved. The scooter rubber brake lines a similar so maybe worth a shot if no joy?
Thanks, the pipes are all about 6 months old so should be fine, the caliper is about three months so i reckon it was not retracting properly due to tight inner seals. I,ll try wiggling of the handlebars, sounds like a plan. I did try to force fluid down from the top banjo where it joins the reservoir but the banjo type fitting is not good for that plan. I,ll crack on and see if i have any luck. Has anyone tried the self bleed system that uses a pump bottle to draw fluid through the system? Might try that for £20 or so.
at work we have a kinda vacuum bleed thing that works off the airline, you just put it on the bleed nipple and turn it on, then keep topping up the master cylinder for about a minute. but sometimes like i said, the air will get a little trapped around the banjo bolt....
Sounds like the caliper pistons are pulling back as far as they are pushing out. Take the caliper off the bike and have a look whilst squeezing the lever. If they are getting sucked back in too far as you release the lever, it's normally caused by the dust seal rolling over a little. You get white crap build up behind the dust seal that is a bitch to clean out and it causes the seal to protrude too far out the slot, grabbing the piston too hard, rolling up, and then releasing as you let go of the lever. It means doing the job twice but try this: Remove the caliper pistons, remove just the dust seals, leaving the fluid seals in place, replace the pistons, then try bleeding the brake again. It worked on my zed when I had the same problem.
I get that. Would i need to replace the dust seals at some point though or just run with the fluid seals only? Would the brake dust not get in and contaminate the cylinder bore?
You will have to replace the seals, both fluid and dust, this is just to identify the possible cause, taking into consideration all you have already checked.
Stripped it down, cleaned and lubed the pistons with brake fluid, rebuilt and it bled properly this time. Possiblyan air lock possibly sticky piston? No idea but solved.
