Be careful using brake fluid on anything.
My guess it that neatsfoot oil should work just fine.
From an article by Bob Adler in October 2004. Vintage Truck:"Make Components Last Longer"
"Vacuum wipers aren't known for Herculean torque, but you can grow fond of the hissing and flip-flop sounds vacuum motors make. Periodically, squirt some motor oil into the wiper-motor air intake (a pinhole near the valve-tripping mechanism, not the rubber hose going to the intake manifold) as the wipers are operating. This will help to seal the moving paddle to optimize power.
Tim
