Hello Glen, priming: A couple of reasons for priming are the type of water used and, possibly, surging were water is on "carry over" into the main steam line. When water is heated, many insolubles will be precipitated, the more heat and the hotter the water - the more these chemicals appear. If the density of the water is allowed to increase - these precipitates will cause foaming which will be carried into the cylinders. Hence your boiler treatment and regular blow downs are required. The main purpose of the boiler treatment is to keep scale from forming (from the hard precipitates) and as an oxygen scavenger. Free oxygen is highly corrosive in boilers. Surging, caused by rapid acceleration or stopping can cause water to be "lifted" through the regulator into the main steam lines, but this is usually rare. Overfilling the boiler increases the likelihood of priming. Driving traction engines (all types) priming may occur when going down steep hills, fortunately the short exhaust lines mean that most water is blown straight up the chimney to fall where-ever. Back to the density of boiler water, the following figures are taken from "Locomotive Management 8th Ed. (8/- cost) of 1942 PSI 20 Temp Deg.F 227 Density(lb per cu.ft) 0.05 Vol (1lb in cu.ft) 19.9 40 267 0.09 10.3 80 311 0.18 5.4 160 363 0.35 2.8 Thus all steam engines (some of which may have steam heating on the cylinders) will always use the drains (open) to get rid of any condensed water. Steam you can compress - water you can not - it has the un-loved habit of blowing cylinder ends off etc. Trust this helps Bob