Hi I am new to this site, so I apologise if someone else has mentioned this. I see there is a very comprehensive thread about the building of Stuarts steam feed pump. There are some contributions mentioning trouble in running the pump. I built one about three years ago, and, just as I was warned it would not run on steam. It did however run on air! Hydraulic lock! If made to the drawings, at the ends of each stroke, the piston covers the holes leading to the drain cocks. I carefully filed semicircular grooves from the ends of the bore to the drain cock holes. On reassembly, it ran "like a Swiss watch".
To start it, the drain cocks are opened and the steam turned on. Steam issues from one drain cock. This cock is slowly closed until the piston travels to the other end. Steam now issues from the other drain cock. The remaining cock is slowly closed until the pump starts. It is allowed to run with the cylinder drains partly open for about half a minute. Once the pump is hot, the cocks are fully closed, and the steam supply adjusted to attain the required stroke rate. I hope this may be of some help to someone.
Cheers