Yeah, the reservoir has two chambers - one for the front brakes, one for the back brakes. The front brakes get the front 90% of the reservoir, so it's tough to see whether there's any fluid in the back portion. Hold your shop light right up to it.

The other guys are correct, you need to fill it all the way up to the neck to insure the rear portion gets fluid. I think you can empty the rear with about ten vigorous pedal pumps.

Don't ask how I learned all this

- JimY