be so small that it serves essentially only as a seal for the clearance situated between the piston 8 and the cylinder 9 these occurrences ensure a closing of the suction nipple valve 4 without an undesirable intake of air after the suction nipple valve 4 is closed and the air entering behind the oil has displaced the oil situated between the piston 8 and the cylinder wall 9 an airing of the pump chamber occurs the operation of the suction nipple valve control is independent from the presence of the oil filter 19 that is even in an oil circuit without an oil filter as symbolized by the broken line bypass 20 the suction nipple valve 4 and its control operate in a satisfactory manner a particular advantage of the construction of the suction nipple valve 4 and its control operating as a function of the oil pressure resides in that both cylinder and piston arrangements 8 9 and 22 23 are because of the desired clearance between respective piston and cylinder not subject to strict manufacturing tolerances and therefore are inexpensive to make by appropriate choice of the oil quantities q 1 and q 4 and by a corresponding adaptation of the force of the spring 31 the control arrangement may be adjusted such that even at relatively small pressure drops in the oil circuit for.