[Doublepumper]

In my limited experience, it is either cracked vacuum hoses or gummed up wiper motor internals that cause most of the problems. Sometimes the motors and valve mechanism can be cleaned out of old gunk and lubed to get them to work again. Sometimes it's the paddle seal dried out and leaking inside the motor, which is harder to deal with. I haven't tried it but, supposedly brake fluid can be used to soften the paddle seal and get them to seal again.

Good luck!