Our V16 ALCO engine was recently overhauled and since then we have a problem with a loss of water pressure. The problem doesn't seem to occur at idle and not even at full rpms only when the engine is on load. The jacket water pressure slowly drops until there is no pressure at all. At the same time the level in the expansion tank rises considerably. When load is removed the pressure tends to return and the expansion tank level returns to pretty much its original level. There is obviously air in the water at this point. We have done hydrostatic tests on all the heads, and the after cooler with no luck. The water pump doesn't lose water from its seal when not running so I don,t think it is taking in air while running. Any suggestions???