Friend of mine is using a Case 550L (and has also used other brand dozers in the past) that have the de-accelerator control w/ the foot steering. He has commented that the Case seems to not be DE-accelerating but just ACCELerating instead. That to me sounds like an issue with the dozer...but I have never driven one, Case or otherwise, so I do not know. Does the machine possibly have a 'switch' that changes the functionality of the lever??
Most of the case machines had the accelerating with the older units, the newer units could be ordered with the accelerating or the decal pedal, I don't think you can change it, without parts. There may be a broken spring causing this problem with the pedal but more than likely it was bought with the accelerating type pedal.