The fill volume should be between one and 2 gallons, the complete floor of the dishwasher should be covered with water to at least some degree if you open the door.
If you think the dishwasher is getting too little water, try adding another half gallon of hot water and see how it sounds if it makes a noticeable improvement you probably are getting too little water.
Most likely causes a bad inlet valve on a machine this old.
John L