You're either not getting enough vacuum to the motor or it needs rebuilt. The motor in my 38 has a flapper that hangs down in the center, pivots at the top & seals against the sides of the housing. The flapper moves side to side similar to a steam engine piston. As the vacuum pulls it to one side a mechanical valve changes the vacuum to the other chamber and it gets pulled back the other way. You can get a fuel pump that has a vacuum pump built into it to assist the wiper motor. You can replace the vacuum motor with an electric but I like to think of my vacuum wiper motor as the original "delay" wipers.
