As was mentioned you are most likely pulling in air at one or all of the large suction line fittings. There are rubber seals in each one. You might get by with just tightening them all up but they are 70 years old now so you might want to think about replacing them. They aren't very expensive from Deere.
Another possibility is it's bypassing inside. Using the wrong oil will make it worse. I use 20 or 30 wt non detergent motor oil. It won't foam, works better and leaks less than anything you can put in there. It's in the motorcycle section at my local Advance auto parts.