the small shifting lever, the one that shifts to smaller cogs, is loose. it moves about 1/4 inch from front to back. it has just started to do this in the past few weeks. it still shifts fine but im worried about it breaking inthe middle of a ride, can this lever be tightened or replaced or will i need to buy a whole new shifter/break lever. which bing me to my next question can i buy just one? ive look on the net but all the places ive seen only sell both shifters.