I have a Craftsman Pro Series lawn tractor that was built by MTD. I started getting very low power in reverse. Forward was fine. In my case, it was air bubbles in the hydrostatic transmission. To cure, first jack up the rear axle until the wheels are off the ground. Then pull the transmission bypass rod out. Next, crank the engine and release the brake. Keep the throttle at low rpms. You will then depress the forward pedal for a few seconds and then the reverse pedal for a few seconds. Do this three or four times. Then push the transmission bypass rod back in and alternate forward and reverse several times. Shut down the engine, lower the rear axle and test. Reverse should be much stronger now.