Carl was more on the right track in how diluting with "neutral" water affects pH. If there were no pH buffers in the water, then diluting by adding an equal amount of water at pH 7 would roughly cut in half whatever was in excess in the water you were measuring -- in this case an excess of hydroxyl ions since the pH was above 8. pH is a logarithmic scale of the concentration of hydrogen ion and that concentration is in inverse proportion to hydroxyl ion. So in this case with a pH of, say, 8:

pH = 8 = -log10([H+]) = -log10(10^(-14)/[OH-]) = 14 + log10([OH-])

so [OH-] = 10^(-6) so cutting this in half would be 5x10^(-7) which would result in a pH of

14 + log10(5x10^(-7)) = 7.70

In general, diluting in half with water that is a pH of 7 will result in moving the pH towards 7 by about 0.3 units (-log10(0.5) = 0.3).

As for what happens when there is a pH buffer, the dilution cuts all the chemical species in half (assuming the diluting water has nothing in it) so the carbonate buffer resists the change in pH so the pH change might be even less.

The bottom line is that you can't use dilution for the pH test.

Richard