[medra42]

So.. I noticed the other day that the rear brake rez was just above empty. I dumped fluid in, bled it through.

I've been checking it every other day since then and.. almost every time I check it almost or empty, so I bleed it and get the bubbles out again.

I don't see any leaks and I never see any puddles. What gives?

[Arizoni]

When you bled the fluid, did you pump enough of it thru the system to get all of the air out?

It should take over 1/2 cup of fluid to make sure the system is air free.

Also, remember, shaking the can of brake fluid before putting it into the master cylinder will put air into the fluid.  This air will cause the brake to be spongy.

Once the system is bled, the only thing that could cause the fluid level in the master cylinder to go down would be a leak or wear on the brake pads. 

[gashousegorilla]

So.. I noticed the other day that the rear brake rez was just above empty. I dumped fluid in, bled it through.

I've been checking it every other day since then and.. almost every time I check it almost or empty, so I bleed it and get the bubbles out again.

I don't see any leaks and I never see any puddles. What gives?

  No leaks at the caliper pistons around seals ?

[medra42]

I'm not seeing any leaks anywhere. I'm going to look a bit closer today. It's got to be a leak somewhere.. just cannot find it.

[medra42]

I've found some liquid residue on the underside of the caliper.

How can I go about finding and addressing a piston leak?

[Guaire]

New seals

[Arizoni]

If that's the problem and your really losing fluid like you say you are the area should be wet.

Anyway, the source of the leak could be the brake bleed valve is loose.
It's the small hex fitting with the small rubber cap on it.
Under the cap should be dry as a bone. 
If it is wet, or the area where the bleed valve threads into the caliper is wet, tighten the valve up.

It could be the banjo fitting where the hose connects to the caliper is defective or loose.  Both the upper and lower surfaces where it meets the underside of the bolt or the caliper should be bone dry.

It could be just a bit of chain lube or oil that splashed on the caliper.
Oil or grease feels slippery on your fingers.  Brake fluid does not.

It could be the caliper piston seal(s) leaking.

Although I think  there are repair kits which provide the rubber seals available, unless you have done this sort of repair, it's best left to a mechanic who knows what they are doing.

Your CGT should be covered by a warranty that will cover any repair to the brakes so I really recommend that you take the motorcycle to your dealer and let him fix it if the problem is with the piston seal(s).

While your looking around, also check out the rubber bellows on the rod that drives the master cylinder.  It isn't unknown for the master cylinder piston seals to develop a leak.
Again, if it is the master cylinder seals, let your dealer fix it.

We are talking about a life/death safety issue here so don't try to do things that are beyond your experience level.

[medra42]

Thanks for the tips. I re-bled the brakes tonight. When I started there were still some bubbles, so I hadn't bled them as well as I should have. At any rate, I verified no bubbles this time 'round.

Tomorrow morning I'm going to go out and make sure everything is dry, then ride it around the block a few times, see if I can find any liquid.

I'm curious if it isn't the bleeder valve. It's not loose. But the damn thing was so tight when I went to bleed the brakes, I'm curious if the thread didn't get stripped when they did it. If that's the case, does anyone have tips on sealing those threads?

I looked at the master cylinder, and I don't see any wetness there, so I don't think that's an issue. I checked line connections and banjo bolts.. don't see anything there.

[medra42]

OK.. so.. scratch everything I wrote. I went to take a quick look at the bleeder bolt and I gave the pedal a few hard pushes to see if the threads there were leaking.

What did I see but brake fluid dripping from the center of the bike, onto the frame, then onto the chain. So the leak is in there somewhere. I'll have to get the rear air box cover thing removed tomorrow and take a look behind.

[medra42]

Source confirmed gents: rubber hose from rez to master cylinder got chewed up somehow. Probably happened when I was replacing the back wheel. Guess I pulled on something just-too-hard-enough.

[Arizoni]

The rubber hose from the rez to the master cylinder is not under any real pressure but if it is damaged, as you've found, it can leak.

The only problem with replacing it is it has to be the right kind of rubber.
Some of the rubber hoses, like those made with nitrile (which works with gasoline) will rapidly fail if they come into contact with DOT3 or DOT4 brake fluid.
Not only will they fail, the wrong kind of rubber will contaminate the brake fluid which can cause more problems.

[medra42]

Any tips on buying the hose. No one has any lines long enough. My choice seems to be relocate or try to get through the dealer.

[Farmer_John]

Any tips on buying the hose. No one has any lines long enough. My choice seems to be relocate...

I don't think moving would solve this issue...

[medra42]

I could cut the torn hose to length and use the good bits. 

[Farmer_John]

I could cut the torn hose to length and use the good bits. 

Nah, use this:

http://www.yoyodyneti.com/category.aspx?categoryID=3013