It is important to stick to the recepie of the Master Cleanse.
The maple syrup is not in the recepie for sweetning purposes. It is strictly for nutritional purposes.
Grade B syrup has the highest mineral content which is necessary to make the cleanse work. Without it you could wind up deficient of nutrients.
A while back someone was posting here and reporting some aweful problems while doing the Master-Cleanse and how horrible it was to do this cleanse.
After a few days it turned up that she was replacing the maple syrup with a natural sweetner - agavee or something.
Often times its just better to stick with what is known to work.
The Master-Cleanse by Burroughs does just that.