More brakes

[stubmw]

Hi all,
I've  justgot back from a ride out and noticed a slight binding on the front disk. On further investigation i see some small weepage at the handlebar housing/MC joint and when i unscrewed the MC cap there was a small release of pressure and the wheel became free. I guess i need a rebuid kit?? Any ideas?
Question is, i have a 14mm master cylinder, single disk, ate caliper and square reservoir, so  should i be looking at getting a 12mm mc (after reading a recent post on brake upgrades with accompanying ratio chart) or rebuild the 14mm. What was the 14mm MC designed for? The brakes have never been brilliant since i bought the bike
 this year.
Stu

[Barry]

What you have isn't stock as an Ate single disc would have originally been matched with a 13mm round master cylinder. 

A 14mm master was never used with a single disc as it gives a ratio of 13.25:1 which is pretty terrible.

While the master cylinder size needs to be changed I think the fault might be in the cap or rather the rubber seal under the cap.  Can't say I've looked on a rectangular motorcycle reservoir but on my round one there is a small breather slot in the rubber gasket to allow the reservoir to breath.

[Bob_Roller]

With your brake binding, this comes up every once in a while .

With the mster cylinder cap removed, there are two ports at the bottom of the reservoir drilled into the metal of the master cylinder, one is  large hole, it provides fluid to the master cylinder piston .

The second looks like a conical depression, there is a small hole at the bottom, if this hole clogged, it won't release all the pressure in the brake system when the lever is released .

I would make sure this hole is open before any more troubleshooting .

[nhmaf]

+1 with Bob R's diagnosis - this sounds  much like the blockage of the small m/c ports - it takes very little "crud" from compromised brake fluid to block them up and cause brake drag.

[Justin B.]

From what I can remember there is no "vent" in the rubber "bellows/accordian" thingy.  When the fluid level drops it follows right along so that there is not a huge air-space above the fluid.  At least that's how I envision it working...

[stubmw]

Thanks guys for your suggestions.
I found the vent slot at the bellows, that was clear and from what i can tell seems to balance the pressure above and below them.
Poked some fine copper wire down the two holes in the bottom of the reservoir and they are clear. BUT when i operate the brake lever without the cap on there is a steady stream of bubbles being drawn into the reservoir from the larger of the two holes which is pressurising the system!?
The brakes are definitely not spongy, so i guess its drawing it in on the return stroke. Back to a rebuild kit idea?
Stu

[Barry]

The bellows couldn't allow for movement in the fluid level unless there was a vent otherwise a vacuum/pressure would be produced above the bellows.  The slot I mentioned and that Stu was able to find is a vent to the airspace above the bellows so it allows the bellows to expand and contract with fluid movement without allowing the fluid to leak out. It's a clever solution, take a look next time the cover is removed.

The bellows must have been fully compressed upwards to lock the brake on until the pressure was released when Stu removed the cap.

[Matt Chapter]

BUT when i operate the brake lever without the cap on there is a steady stream of bubbles being drawn into the reservoir from the larger of the two holes which is pressurising the system!?

You've got a leak somewhere, or never bled the brake system after changing a component.  The air is trapped in the lower part of the system, i.e. caliper, hoses, splitters, etc, and as you use the brake lever to push fluid down, air comes up.