If you find that your expression pedal is acting strangely, with all of the value change bunched up on one end of the pedal, it's likely a polarity issue. There are two different polarities of expression pedal, and if you have the wrong one, those are the symptoms you would see.
Mission Engineering makes a pedal for us that's specifically designed for our products called the SP1-RJM, but most others will work, either with a switch setting or a special cable. You can find more information about expression pedal compatibility, including a list of pedals known to work, in this article: What kind of expression pedals can I use with the Mastermind GT/LT/PBC?
You can buy a polarity reversing cable from custom cable builders like Best-tronics: https://btpa.com/si17-xx.html or Alphatone Audio (be sure to select the Polarity Reverse option).