My students and I rebuilt two of mine today. Both were frozen up and had all kind of crud in the intake lines.
The first one we did took alot of cleaning to get to work. Oil, grease and a paper clip to remove some kind of bug got it working. But, it was not going full sweep and only worked at lower pressure from the air nozzle and shop air. I was just glad it worked and we moved onto the second one.
We got it going in a lot less time since we knew what we were doing. We also left the cover above the shaft off when we tried it. It went real slow, but it went. A little air tool oil around the shaft had that thing moving like a sewing machine needle. Full sweep too.
We pulled the cover off the first one and put more oil there. Same result. Full sweep and very fast at all air pressures.
We put the covers back on and the sweep reduced. That was when I noticed the stops inside the cover. They limit the sweep and have threads on them. I didn't see a way to turn them besides pliers, but if you have the old metal style of wipers, pull the top cover off and see what kind of sweep it gives you. Then turn the screws or grind them down until you are happy.
I left mine alone since they are off the windshield frame, the windshield is off the cab and the cab is off the truck. I have never seen them work on a truck and have no clue if I will like the movement they have now.
I didn't see the button Chris was talking about. So, there are at least 2 different styles out there and this may not apply to yours.