06 Beta Front Brake Problem

[bowellsyboy]

Hi, My front brake has stopped working. The lever comes all the way back to the handle bar grip and the adjuster screw is right in as well. It was fine when i was out on it last week and seems to have happened over night.

Tryed to bleed it just now, but when i pull on the lever nothing comes through. I think its the master cylinder but am not sure. Also, if it is, how do i go about changing it. Ive never worked on brakes before so dont really know what im doing!!!!

[tombo46]

I would say the master cylinder seal has something wrong with it. either than or or the seals around the pistons in the calipers could be worn.

Tom

[stork955]

Gday. Back that adjuster off! You must always have clearance between it and the m/cylinder piston. What happens is this -

The m/cyl has 2 holes in it, one is tiny and is called the transfer port. The other is large and is the main supply port between the reservoir and the actual cylinder. As the piston moves forward in the bore it closes the transfer port with the primary seal (the "front" one).

The the supply port remains open behind the secondary seal (rear seal). Then pressure is generated and the brakes apply. If the adjuster is wound in it generally closes the transfer port which should be open while the brakes are "at rest". This is to allow fluid to easily pass between the cylinder and reservoir when the brakes heat up and the fluid expands, or to allow fluid to enter the system to compensate for pad wear. The fluid normally passes backwards over the primary seal from the supply port when the piston is released. A one way type of system if you like. If the transfer port is closed continually the amount of fluid in the system cannot change as pads wear or heat is generated, and the master cylinder will never self-prime. Usually one of two things happens next - either the lever goes spongy, or the brakes self apply. Neither is good. So - back off the adjuster until there is a small amount of clearance, say 0.5 mm, gently pump the brakes a few times while checking fluid levels - dont let it run out! You may find the brakes return on their own. If you have adjusted this to fix another problem in your brakes back it out again and go looking for the real problem.

Hope this makes sense,

Cheers,

Stork.

[bowellsyboy]

Just been out in the garage for the last 2 hours and have finally got it sorted. I replaced the piston in the master cylinder, set the adjuster right out and bled the system. Tryed to bleed it the normal way but couldnt get a good pressure, felt spongy, so after swearing, cursing and kicking of the bike did what you sugested heath and pushed it from the bottom up with a syringe. Worked a treat. Feels better than ever now.

Thanks for all your help guys.

Mal