Lost Brake Fluid Mystery

[ToyOdie]

I noticed the other day that my rear brake pedal was going down farther than normal. I changed my pads abouth 6000 mile ago so I thought I was due for new ones. When I did a visual check by removing the inspection cover, it looked like they were getting close enough to change. I went out and bought some new ones and started to change them. When I removed them, I discovered that I had at least half of the pad left. The outside pad had worn slightly more than the inside so I decided to swap them. In the process of compressing the pistons, I decided to check the fluid level in my reservoir and found it EMPTY! I finished installing the pads and added fluid to the reservoir. The level must have been just below the bowl because it I got a few bubbles come up while filling and when I blead the breaks I didn't get any air from the caliper. I ran enough fluid through the system to ensure that ther wasn't any air in it. My pedal is tight and working perfectly. Now the mystery begins.

 

I inspected the entire rear brake system to figure out where my leak was (I surly had to have a leak). To my surprise I couldn't find any evidence of a leak or even dirt collecting where the had been a leak. I've been working on cars, airplanes, and bike most of my life and have never had brake fluid just disapear. Hydraulic fluid doesn't evaporate (as far as I know).So where did it go? I know that the reservoir was full the last time I installed pads and I have checked the site window ocasionally when I was cleaning the bike. Has anyone else seen this or have a resonable explaination?

[RandyR]

when pads wear, the pistons are more extended. that accounts for some of the fluid.

 

I'll note also when filling with new pads in place, that the pistons may be back farther than needed until the brakes are used a few times and the new pads settle in. So maybe they weren't filled as much as you thought.

 

This may or may not have anything to do with what you're seeing.