Hi guys.
Put my 125 through UK MOT. Failed on a few things.
Then main problem is my front disc brake. I bled the system to put new pads in and unfotunately got air through the master cylinder. One of the 2 caliper pistons was ok to push in, but the other was not easy at all. Finally got both to 3 - 4mm still showing. Tried a normal refill and slowly found it wouldn't take fluid back into the brakeline. Blew through hose part with mouth and its quite clean and free. Knew the master cylinder to be ok, or so I tought.
Struggled like hell over 5 days to remove both pistons to check caliper chambers. Finally got them moving with pliers.....Hmmm, a little damage, but mostly on top edges, so not a real problem A little muck in chambers - cleaned. Rubbers were nice, but I did put a couple of scratches on sides of pistons. Anyway, cleaned pistons and they went back in as expected, more force to pass the 2nd rubber seal on each piston. I set them to about 4mm showing to gets pads in and caliper onto rotor.
After watching Youtube, I did a reverse bleed with a large syringe and see through bleedline. After an hour, I could only get fluid back upto top of brakeline, (undid it to check). Realised that the smaller return hole in master cylinder was maybe blocked. I poked needles etc, anything, the unblock it. Washed it through many times with just water. Working the lever, I finally got bubbles from smaller hole! Mounted master cylinder back in position and continued reverse bleed till I got a tiny bit of fluid covering both holes in master cylinder. Would not fill anymore, so I topped it up.
Then I switched to the normal bleed process. Worked lever fast to get vacumn, kept lever pulled in, undo bottom nipple 3 seconds, few bubbles, fluid etc, tighten nipple....repeated many many times! Bleed line was always sunk in old fuid to stop air getting in system. I CANNOT GET PRESSURE ON THE BRAKE LEVER AT ALL! Brake fluid is getting through because a little is coming out eachtime I open the bleed nipple. Also tried slow pulls on lever whilst watching for air bubbles.
ANY IDEAS GUYS??? IS IT LOOKING LIKE A WEAK MASTER CYLINDER?