12v is only connected through the fuse to the valve when the ignition is off. The oil pressure switch only connects an earth to the other side of the valve when the engine is spinning and generating oil pressure. With a running engine the valve only operates as the ignition is switched off, and when the engine stops spinning and oil pressure dies away the earth is disconnected and the valve releases again, usually with an audible 'click'.
The anti-runon valve. 'A' is the fresh-air inlet from an open-ended hose either by the chassis rail or through the radiator diaphragm; 'B' is the hose connection to the bottom of the charcoal canister; 'C' is the small-bore hose connection to a port on the inlet manifold; 'D' are the electrical connections:
The anti-runon valve (A) and oil pressure switch (B) as fitted to Bill Etter's 76:
