Check Valve - The most common check valve in use on steam tractors are known as swing checks, and the most common location is between the boiler and a feedwater device. They have a hinged valve that is free to swing away from the seat when the flow is from the seat side, but swings shut tight against the seat when the flow reverses (this makes it the plumbing equivalent of a diode, for those of you who are into electronics). A misaligned or warped seat, foreign matter, and scoring can cause the valve to leak, allowing hot water and steam to flow back into the injectors and pumps, but on the whole they tend to be fairly trouble-free.
    Most good check valves are built so that they can be reground as needed, and some are built so that the valve spins slightly with each cycle, preventing asymmetrical wear. Some of the cheaper check valves are built with one-piece swings, and are not able to be resurfaced - these should be avoided.