I would replace the diaphragms. Often there is a tiny hidden crack near the center where the bolt goes through. If you can, tilt the pump for 10 minutes to remove the old oil. If not, I usually clean the old oil out with a paper towel. Cpmpressed air is a possibility--but messy--you would have to wash your whole truck, LOL!
Don't install the diaphragms upside down, and be sure they are seated in the grooves for a good seal.