Hi Darren
I have been driving my CMPs for years and tried to get the vacuum wipers to work or work better for years. The flat fact is that vacuum wipers only work when going down hill, just not enough vacuum to make them work. Even re-engined with 261cu.in. which is producing good vacuum (5-10 inches) on the flat even NOS vacuum motors that have been cleaned and relubed with the proper wiper grease only work lazily.
But I've found a better solution is to treat the windows both sides with Rain-X. Even when driving in heavy rain the reverse-slope windshield will stay surprisingly clear. Here is a video of driving in the rain in HUP (OD)
https://youtu.be/wruelyqEHBc
I'll have to find a video of showing the effect of Rain-X in my Patter 13 C60S on the highway in a heavy rain. The Rain-X seems to work better as the speed goes up. Only thing is you do have to clean the windshield of mud and dust to get it to work best. Generally I reapply the Rain-X if I know I'll be driving in the rain.
Cheers Phil