I was able to find some sufficiently small hose clamps that fixed my air leakage problems, but now I'm having trouble with the accuracy of the pressure sensor. Here is some of the data I got during my last test session:

SG test solution -> ADC counts (0-1023)
1.000 -> 220
1.100 -> 240
1.062 -> 237
1.028 -> 233
1.000 -> 227
1.100 -> 244
1.062 -> 237
1.100 -> 242
1.000 -> 223
1.000 -> 226

So as you can see the trend is generally correct but there is too much hysteresis in the values. After perusing the datasheet for the sensor it seems that the overall accuracy is +/- 5% worst-case, so the sensor is performing much better than its spec. But it's still not very useful for my purposes.