125-200cc Rear brake master cylinder problem

Guys I lost rear brake pressure (Wr125) and seems like cylinder doesnt build pressure. Piston rod thing seems to just hit metal . Very light brake pressure at first then ok again and then no pressure.
Tried bleeding air from fluid . Doesnt seem to be any air.


Dropped the cylinder to a mechanic and he fitted an internal piston kit with seals . Not sure if this was exactly the right part. From reading other posts it may be hard to get genuine part .
Still has the same issue. so seems like that wasnt the issue .
Must be common though as a 09 te310 I just picked up has the same issue
Any ideas ? I cant think what has caused this .
Is there a good method for filling fluid and bleeding for pressure ?

bleeding brakes on these is a trick
if you have air you need gravity to be your friend, either remove the master or the slave cylinder
the issue is if you have air it is at the highest point, if the master is not there then you need to get it there
once you have it at the top open the bleeder, keep the reservoir full and the gravity will flow fluid
it there is still air, crack the line at the master, push the pedal and hold, then close the line
the secret is getting the trapped air that is in or at the highest point

Can you post a picture of the stock master cylinder on your 125? I might have one that I can send to you. I know I have oem rebuild kits too. Of course I have CRS so post a pic and I will look.

I used a syringe to force new fluid in at the caliper blead point . I then tried normal bleading method. It doesnt seem to allow fluid up through to the cylinder . Doesnt push fluid through to reservoir. Not sure if this is normal ? There is no separate reservoir other than in the master cylinder . I needed to fill it as well. No bubbles . Cant get pressure .
I might try removing it and lowering it as you say see if any air comes to top

Walt pic attached . I will PM you thanks

the short answer is unless you have a way to pressure feed the fluid to remove the air you need gravity work for you
just replaced my front brake line and to get it to work used gravity to fill the line by elevating the master to fill the line via opening the bleed screw on the caliper
then bleeding the top by pressurizing, as in pulling the lever then cracking the line where it comes to the master
3 stokes and I had pressure at the lever

I use a vacuum bleeder to draw the fresh fluid through. I find the master cylinder seals resist back pressure so it’s easier to pull the fluid through in the normal way. If you do get air in the system then remove the caliper and lift it higher than the master cylinder. The air bubbles will rise to the caliper end and can then be bled out.

I removed caliper and raised to seat level and connected a line with fluid in it and had it above caliper. Kept pumping pedal and bleeding . Found the air . Started drawing fluid from bottom master cyilinder . Had to top it up again . Eventually I got some pressure in pedal .
Yay

Thanks guys .

the important part is you got it done
but the lesson is a straight vertical path with no high area to capture air
wether you raise the master or the caliper is your choice