Your best way to improve the performance of vacuum powered wipers and other things like 2-speed rear axles, etc. is to install a reservoir tank. Buy a piece of 3" PVC drain pipe and a couple of caps, glue them together, and hide about 3 feet of it inside a frame rail under the cab. Put a rubber grommett and a one way check valve like a brake booster uses into one end cap, and install a screw-in barbed vacuum hose nipple beside it. Then when the vacuum drops, you'll have enough reserve to keep the wipers from stopping. It will also be MUCH less expensive than buying an electric pump.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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