within the cylinder head cover 4 through the outlet valve 19 the air is then sucked from the brake booster not shown through the pipe 16 and the inlet valve 18 into the pump lower chamber 15 at the same time the lubricating oil which has contacted with the outer peripheral surface of the roller 23 and splashed is introduced into the pump lower chamber 15 through the communication holes 28 the bearing opening 24 the gap between the bearing 11 and the piston rod 10 and the oil passages 12a by the negative pressure of the pump lower chamber 15 then being dispersed within the pump lower chamber 15 by repetition of vertically reciprocating movement of the piston a sufficient amount of the lubricating oil flows into the pump lower chamber 15 and adheres to sliding surfaces of the piston ring 9 and the cylinder 5 so that a good sliding condition between them can be maintained when the piston 6 moves downwardly the air within the pump lower chamber 15 is discharged into the space within the cylinder head cover 4 through the outlet valve 20 the air is then sucked from the brake booster through the pipe 16 and the inlet valve 17 into the pump upper chamber 14 the lubricating oil adhering in the vicinity of.