On my 49 I did what Frank50 suggested but used mineral oil. That was three years ago, they still work and we get a LOT of rain; I try and avoid driving it in the rain but they get used frequently regardless (weather can change on a dime up here). If it failed I'd probably send it out for a rebuild and keep vacuum just for the fun of it. I added a vacuum cannister but not sure how much that helps.
That's not to make it sound like they work as well as any modern system - no they don't
But they keep the visibility (within the reach of the wiper).
Use OEM wiper arms & wipers if you stick with vacuum.
I believe that your last line is spot on.
Aftermarket wiper arms come with a spring which is too strong, causing to much friction on the windshield even when it is wet, for the vacuum motor to work properly.
We need a spring genius among us to come up with a proper spring which can be swapped out for the too-strong one.
Alternately, maybe there is a way to install a short link to the too-strong spring to reduce the overall force placed upon the wiper blade.
