Bent brake rotors causing you troubles? Mildly bent brake rotors are easy to fix, and it's something you can do in your own garage. Telltale signs of a bent rotor are dragging brakes, spongy braking, or one front wheel locking up before the other (the rotor of the wheel that locks up should be good, so investigate the other rotor).
To inspect for a bent rotor, take the wheel off and pry the brake pads open so there is a visible gap on both sides of the rotor. Spin the hub around while watching the rotor. Look for the gap to close up against the brake pad. Once you spot the pad closing against the rotor, mark that area with a Sharpie marker. Now put a large crescent wrench on the rotor where the mark is and bend it slightly away from the pad. Roll it around and check the gap again; it will likely take several attempts to get it straightened out completely. When the rotor is straight, the brakes will work better and your quad may even be slightly faster, as rolling resistance will be eliminated.