You haven't indicated what keyboard you are using and what functions you are using the expression pedal for. I use a Yamaha FC-7 pedal as an expression pedal to control a hammond module. A hammond organ has a pedal and to mimic what it does you would need expression pedal functionality. I find that with the FC-7 I get the same long sweep that I get when I'm using a real hammond. Using a MIDI scope I can see that the sweep is linear, i.e. 0-127 in a linear fashion. With some pedals you get you get 80% of the increase in the first 25% of the travel of the pedal. Based on what your wrote it seems as though you are getting the opposite effect, 80% of the volume in the last 25% of the pedal. I would research this pedal. It's construction is not plastic, it is very sturdy; I've had mine for years and it still works great...
One of the keyboards that I currently own, for each program in the keyboard I am able to configure the pedal to be either an expression pedal or a volume pedal. So I could use the FC-7 to be a volume controller on one program and an expression controller on the organ programs while plugged into the same input jack on the keyboard. Might want to ensure that your program configuration on your keyboard is also correct.
One other thought, in some cases (this also applies to the FC-7 as well) the polarity can be backwards on the pedal and incompatible for your keyboard. When it is it can cause the pedal to react as I mentioned earlier. So what you could be experiencing is a polarity issue. See this article regarding the simple fix:
http://www.soundonsound.com/forum/showflat.php?Cat=&Number=322584
and this is where you can purchase the simple fix:
http://www.music.ashbysolutions.com/misc.htm
