Interesting subject: If you get a bit of air into the cylinder of your water pump there is the chance of the air bubble acting as a spring. I.E. on the force stroke the air is compressed and then expands again when the ram is moved back. Result: no water is pumped if the air pressure is not high enough to move the discharge check off the seat because the check at the boiler has allowed some water pressure to fill the line. Solution: To avoid this pump rams should have a pin or area at the working end that fits fairly close to the opening in the valve body. This is called of all things "Anti air lock pin" When running my engines I always roll it along the track to make sure the axle pump has been purged of air by leaving the bypass open and seeing that water is ejected from the overflow line back to the tender or tank. With the hand pump (emergecy use only, and test) If any problems appear I loosen the nut at the boiler feed check and make sure water, not air is being discharged, then tighten the nut again of course. On larger locomotives water should be introduced somewhere near the front of the boiler. The feeding of cold water right over or (Heaven's forbid) onto a hot crown sheet can be a route to failure. Of course this does not apply to injector feed with hot water. John Meacham California high desert
