Drum Brake Issues

I hope someone can help me out here , my grandson has a grizzly 125 , and it has drum brakes front and rear and im hoping someone has come up with a way to make them work . They work great until he gets in the mud or water and then they barely work at all . As far as i know his dad has changed all the cables and the brakes , and seals on the drums look good . Any ideas to try ? Thanks .