A pneumatically operable fastener-driving tool and seal mechanism assembly, and a method of operating the same, comprises the use of a single, annular seal member, having a substantially V-shaped cross-sectional configuration, disposed within an annularly recessed portion of the fastener-driving tool piston in such a manner that the open region of the V-shaped annular seal member is disposed toward the relatively high-pressure upstream end portion of the fastener-driving tool cylinder, while, conversely, the closed region of the V-shaped annular seal member is disposed toward the relatively low-pressure downstream end portion of the pneumatic tool cylinder Accordingly, the relatively high-pressure air acts upon the radially outwardly flared lip portion of the annular seal member during a power stroke, while relatively low-pressure air acts upon the radially outwardly flared lip portion of the annular seal member during a return stroke, whereby only a single V-shaped annular seal member is required to achieve proper sealing performance characteristics throughout an operative cycle of the fastener-driving tool.