Should you find that your brake lever is becoming spongy, this often implies that the brake system has air in it. This is normal in use and can be rectified by bleeding the brakes. Most of the unicycle disk brakes are now shimano models, but the technique is similar across brands, but be sure you get the right fluid.
There are lots of great instructions online how to do this. Bikeradar.com has done a great blog how to do this
If you prefer a video then they have a video version of the same instructions.