Binding brakes and damaged mastercylinder

[NorR65]

My brakes was binding and I could not press the brake pads back in the caliper. I tried to open the small hole inside the master-cylinder and have damaged a seal or something. it will not build up pressure.
I have decided to bay overhaul kits for both brake calipers and the master-cylinder and new brake hoses. Do I need a new complete master-cylinder since I have damaged a seal or will a overhaul kit do?

[Bob_Roller]

All you need is an overhaul kit .

Remove the master cylinder from the throttle housing, open it up and see if you have any corrosion damage to the cylinder bore, not uncommon to find corrosion in there .

There is a number cast into the master cylinder housing on the outside at the bottom , make sure you get this number, as it is the bore diameter of the master cylinder in mm .

[Lucky_Lou]

I fitted a replacement master cylinder (S/H) as i could not cure the leak on the old one.There was some corrosion in the bore so i used a Dremil with a cleaning brush and a squirt of WD40 worked a treat.
If you have opend up the vent hole make sure there are no burs on the inside or it will score the seal.
   http://www.dremel-direct.com/acatalog/403_19_0mm_Wheel_Shape_Bristle_Brush.html
Lou

[NorR65]

I fear I have opened the vent hole to much.
When I press the brake lever there is no brake pressure and I can see bubbles from one of the holes in the reservoir. Before I opened the vent hole the pressure was OK. I am not sure what I have damaged.
Can somebody explain what the purpose is for the vent hole?
The reservoir was very dirty inside, old brown brake fluid and some thick black dirt.

[Lucky_Lou]

I fear I have opened the vent hole to much.
When I press the brake lever there is no brake pressure and I can see bubbles from one of the holes in the reservoir. Before I opened the vent hole the pressure was OK. I am not sure what I have damaged.
Can somebody explain what the purpose is for the vent hole?
The reservoir was very dirty inside, old brown brake fluid and some thick black dirt.

The main piston seal returns past the vent hole so that as the pads wear and the caliper pistons move out more fluid is allowed into the system to compensate, if this didnt happen the travel on the lever would increase as the pads wear, eventually rendering the lever ineffective as it hits the handlebar.
It may be the inner seal is damaged due to a bur from drilling what size did you open it to?? ive cleared mine before with a piece of fine steel wire you may just need to clean the bore and replace the seal if your lucky.
Lou  

[NorR65]

I too used a thin wire, maybe not thin enough, 0,35 mm.
I did not have to use force, but I pressed the wire 7 mm through the vent hole. I will take the master-cylinder apart and see the condition inside.
Thanks

[nhmaf]

I rebuilt my m/c after I first got my R65 - but there was some pitting/corrosion in the bore which I tried to clean up.   It did work OK after the rebuild for ~1 years and then started leaking and acting up again.  A new m/c (Rectangular type) complete with all parts was only about $150 at the time, and things are great now.  Make it a habit to flush out & replace your brake fluid every 1-2 years and you shouldn't need to mess with the m/c for as long as you own the bike.