If it is setup like my 4035/ML120, there are cables that run from the control lever to the actual valve. Those cables can rust, get dirty, get kinked, and probably a few other things I'm not listing to cause the behavior you're describing. Only real option is to dive into it and lay eyes on the components. Dirt and rust can both be overcome with some penetrating oil and working the control repeatedly (unless the cable inside ends up rusted through and you have no control at all, then it takes a replacement). A kink will require a replacement.