As PILOT designed the CON-40 to hold 0.4ml, a visible air bubble is expected when first filling it straight from the ink bottle. This air bubble will not affect ink flow, especially with the steel balls in the converter, which ensure ink flows optimally when writing. However, you can achieve a fuller fill by priming the feed of your fountain pen. Without taking the nib out of the ink bottle, empty out the first fill of ink by twisting the knob anti-clockwise and refill it once more by twisting it back clockwise. You will find that the ink that remained in the feed with your first fill will displace the air bubble and more ink will be pulled into the converter.