A fastener driving tool, comprising: a working cylinder (41, 44) that includes a movable piston (20, 22), said cylinder including a variable displacement volume on a first side of said piston, and said cylinder including a variable venting volume on a second, opposite side of said piston; a storage chamber that is operable to be pressurized with gases which are not vented to atmosphere after a drive stroke but re-used for a plurality of operating cycles; an end cap that is attached to at least one of said cylinder and said storage chamber near an end portion of said fastener driving tool, said end cap including at least one first gas passageway (62, 64) that is in fluidic communication with said cylinder and with said storage chamber; a movable driver that is in mechanical communication with said piston; a guide body that guides movements of said driver; and a fill valve that is mounted at a side portion of said end cap, said end cap including a second gas passageway that travels between an inner end of the fill valve and said at least one first gas passageway (62, 64) of the end cap, said fill valve being quickly accessible without having to disassemble any portion of a housing of the fastener driving tool; wherein: said variable displacement volume of the cylinder (41, 44), said storage chamber, and said at least one first gas passageway (62, 64) of the end cap all contain the pressurized gases when said fastener driving tool is in use wherein-said pressurized gases are not vented to atmosphere after a drive stroke, but instead said pressurized gases are re-used for a plurality of operating cycles. a working cylinder (41, 44) that includes a movable piston (20, 22), said cylinder including a variable displacement volume on a first side of said piston, and said cylinder including a variable venting volume on a second, opposite side of said piston; a storage chamber that is operable to be pressurized with gases which are not vented to atmosphere after a drive stroke but re-used for a plurality of operating cycles; an end cap that is attached to at least one of said cylinder and said storage chamber near an end portion of said fastener driving tool, said end cap including at least one first gas passageway (62, 64) that is in fluidic communication with said cylinder and with said storage chamber; a movable driver that is in mechanical communication with said piston; a guide body that guides movements of said driver; and a fill valve that is mounted at a side portion of said end cap, said end cap including a second gas passageway that travels between an inner end of the fill valve and said at least one first gas passageway (62, 64) of the end cap, said fill valve being quickly accessible without having to disassemble any portion of a housing of the fastener driving tool; wherein: said variable displacement volume of the cylinder (41, 44), said storage chamber, and said at least one first gas passageway (62, 64) of the end cap all contain the pressurized gases when said fastener driving tool is in use wherein-said pressurized gases are not vented to atmosphere after a drive stroke, but instead said pressurized gases are re-used for a plurality of operating cycles. said variable displacement volume of the cylinder (41, 44), said storage chamber, and said at least one first gas passageway (62, 64) of the end cap all contain the pressurized gases when said fastener driving tool is in use wherein-said pressurized gases are not vented to atmosphere after a drive stroke, but instead said pressurized gases are re-used for a plurality of operating cycles.