I would think a check valve is needed. Think of a long grade going up a mountain. Manifold vacuum is going to drop a lot. You can downshift (4th to 3rd for example) to keep the RPM's (vacuum level) up but the MPH (speed) may be too slow even though the truck is capable of running faster in 4th gear. Also you don't want to overtax the vacuum pump.
Before I got excited and ordered a tank, motor and check valve I would definitely have my vacuum motor overhauled by an expert. The paddle inside the motor has a seal that can wear or age out. The body of the motor can wear inside. There is a top gasket that can age and leak. Adding oil (Trico says 10 wt.) to a wiper motor can make it work better for a while. It's just like squirting motor oil inside engine cylinders and piston rings to raise the compression. It doesn't last if the rings are worn. Also, I wouldn't have high expectations that my $$ investment would produce a clean wipe like a modern truck. Stock design blades and arms can be lacking.
Attached is a picture of a 1947 Kaiser/Frazer vacuum tank booster with check valve. There are similarities to the design of a brake vacuum tank booster (FLAPS) also attached.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
