A pneumatically operable fastener-driving tool comprising: a working cylinder having an inner peripheral wall portion; a working piston movably disposed within said working cylinder along said inner peripheral wall portion; a driver blade fixedly mounted upon said working piston for driving a fastener through and out from said pneumatically operable fastener-driving tool; a source of pressurized air fluidically connected to said working cylinder for supplying working air, having a relatively high pressure value within the range of 551581-827370 Pa, into said working cylinder in order to drive said working piston, and said driver blade fixedly mounted upon said working piston, through a power-driven stroke in order to drive a fastener through and out from said pneumatically operable fastener-driving tool; an annular recess defined within an external peripheral edge portion of said working piston; and characterized in that it further comprises: a seal mechanism assembly comprising a single, annular" seal member, having a substantially V-shaped cross-sectional configuration comprising a main body portion, an open void region, a radially outwardly flared lip portion, and a closed region at which said radially outwardly flared lip portion is integrally attached to said main body portion, disposed within said annular recess defined within said external peripheral edge portion of said working piston such that said open void region of said single, annular seal member having said substantially V-shaped cross-sectional configuration, is disposed toward that side of said working piston which is exposed to said source of pressurized air, having said relatively high pressure value within said range of 551581-827370 Pa, so as to permit said relatively high pressure air to act upon said single, annular seal member, and within said open void region thereof, so as to effectively force said radially outwardly flared lip portion of said single, annular seal member into engagement with said inner peripheral wall portion of said working cylinder so as to ensure proper sealing between said working piston and said inner peripheral wall portion of said working cylinder during said power-driven stroke, whereas during a return stroke.of said working piston, relatively low pressure return air, having a pressure value within the range of 206843-310264 Pa, acts upon said closed region of said single, annular seal member whereby said radially outwardly flared lip portion of said single, annular seal member will remain engaged with said inner peripheral wall portion of said working cylinder so as to ensure proper sealing between said working piston and said working cylinder during said return stroke. a working cylinder having an inner peripheral wall portion; a working piston movably disposed within said working cylinder along said inner peripheral wall portion; a driver blade fixedly mounted upon said working piston for driving a fastener through and out from said pneumatically operable fastener-driving tool; a source of pressurized air fluidically connected to said working cylinder for supplying working air, having a relatively high pressure value within the range of 551581-827370 Pa, into said working cylinder in order to drive said working piston, and said driver blade fixedly mounted upon said working piston, through a power-driven stroke in order to drive a fastener through and out from said pneumatically operable fastener-driving tool; an annular recess defined within an external peripheral edge portion of said working piston; and characterized in that it further comprises: a seal mechanism assembly comprising a single, annular" seal member, having a substantially V-shaped cross-sectional configuration comprising a main body portion, an open void region, a radially outwardly flared lip portion, and a closed region at which said radially outwardly flared lip portion is integrally attached to said main body portion, disposed within said annular recess defined within said external peripheral edge portion of said working piston such that said open void region of said single, annular seal member having said substantially V-shaped cross-sectional configuration, is disposed toward that side of said working piston which is exposed to said source of pressurized air, having said relatively high pressure value within said range of 551581-827370 Pa, so as to permit said relatively high pressure air to act upon said single, annular seal member, and within said open void region thereof, so as to effectively force said radially outwardly flared lip portion of said single, annular seal member into engagement with said inner peripheral wall portion of said working cylinder so as to ensure proper sealing between said working piston and said inner peripheral wall portion of said working cylinder during said power-driven stroke, whereas during a return stroke.of said working piston, relatively low pressure return air, having a pressure value within the range of 206843-310264 Pa, acts upon said closed region of said single, annular seal member whereby said radially outwardly flared lip portion of said single, annular seal member will remain engaged with said inner peripheral wall portion of said working cylinder so as to ensure proper sealing between said working piston and said working cylinder during said return stroke.