My wipers on my '50 Olds 88 will travel about halfway across the windshield and stop.
If I "help" the wiper with my hand it will finish the stroke.
As it starts the next stroke it will stop halfway again.

I checked the vacuum at the wiper motor and I get 17 inches of vacuum.
If I accelerate the engine rapidly, it drops to 10.
If I snap the throttle closed quickly it go as high as 20.
Then it settles at 17 again.
I have the fuel and vacuum pump. Should these readings be higher?
Is that enough vacuum to run the wiper properly?

 

My guess is the vacuum motor has gotten weak. Is it accessible from underhood or do you have to go behind the dash to get to it?

Put some light oil into it and see if things improve. If not the paddle inside is likely worn. I would think rebuild kits are available especially if it's a Trico design motor.

 

Just thinking out loud here: since vacuum wipers are problematic even when in good working order, is it possible to retrofit electric wipers? Is that a thing people do on these old cars?

 

Fusick has re-built wiper motors for $169, I think that's the way I'm going to go.
I may need a re-built fuel and vacuum pump.
17 inches and less under acceleration seems low.

 

Sorry, but I have no interest in electric wipers.
This is an un-restored original car. And I want to keep it that way.