To find a manual you would need the make & model. If you had that you might find one on e-Bay or at a swap meet.
If they are from the thirties they can't be very efficient (even with a modern electric motor). If the owners are concerned with the environment they should get a modern high efficient pump.
Check out tins web site: www.farmcollector.com/Equipment/Pumped-About-Pumps.aspx
The piston pumps are very simple to rebuild and to put back into service, except for the parts that are getting impossible to find. If you can find the parts that they need, then rebuild them and use them. Otherwise, go down to Sears and buy new pumps to replace them. The new jet pumps don't have near the parts to give you problems as do the old piston pumps.
We've also noticed that parts are getting hard to come by. We've considered getting some new piston pumps, but the only company we found that made one that looked good was Dankoff. We're looking into getting a new one this year, but we still like the old ones a lot.