[Zax40]

Check the vacuum line going into the wiper vacuum "motor". Should have vacuum at idle and increase with throttle. Could be the line is old and not sealing or has fell off the inlet. The knob should be cable or linkage activated and move a small slide at the top of the wiper motor. You should be able to hold your finger on it and feel it slide it in and out with someone working the knob to test. Next check the wiper arms that go to the transmissions/towers and make sure they move freely and are not binding. If all that checks out remove the vacuum wiper motor and check and make sure it moves freely. If it is stuck sometimes a couple squirts of WD40 will break it loose. You should feel the resistance of the vacuum but it should not have any sticky spots. If it is too loose your rubber seals may be worn inside.They are not too hard to take apart and clean and inspect. Just be careful with the tiny screws. I used a dollar store baking pan like I do when I take apart carbs so none of the tiny pieces get lost.