I had the opportunity to drain and refill my Hayes HFX-9. I bought the Hayes bleed kit and followed the instructions, but it seemed like it took forever (measured in hours) to get all the air out of the calipers.
Per the instruction, I'd fill the caliper by squeezing the fill bottle for 5 seconds and release. As the bottle returned to it's original shape, I'd see air bubbles being sucked out (this is good). This went on and on, admittedly, the amount of air being removed became less and less. There were times when no more bubbles would appear, then on maybe after the 4th or 5th squeeze, a few bubbles would appear. (It seemed like the calculus problem of approaching zero, but not actually getting to zero.)
All is good now, but after many frustrating hours.
I assume I am doing something wrong. Or is it just the nature of the beast when refilling the HFX-9s? For example, the shape of the caliper housings have pockets where air can get trapped.