The limit valve is for use on steam locomotives fitted with an ejector. It is used to set the highest limit of vacuum used in the system. 15″ of mercury is usually the maximum on most miniature railways.

Limiting the amount of vacuum is important because if the reservoir vacuum is ever higher than the vacuum achievable by the pump or ejector, the brakes will stay on and not come off.

PNP Railways limiting valves are easily adjustable and are small enough to be mounted discretely.

Measurements taken with brass vacuum screw at medium adjustment.