jfnar, I rebuilt my vacuum unit today thanks in large part to Pete52's photobucket pics. I hooked it to the vacuum line on my truck, and it tested great.
Here's the important part: Before I installed the vacuum unit, I tried moveing the wiper arms through their normal range of movement and found that the driver's side had a stiff spot right in the middle of the swing. The passenger side would hardly move at all. Now if I had installed the vacuum unit, I would have suspected that the unit itself was not working right.
So check to see if your wiper arms move smoothly, especially if the truck has set for a few years, or the wipers havn't seen much use.
